As filed with the Securities and Exchange Commission on September 1, 1999.
                                   Registration No. 333-
         -------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                         THERMO INSTRUMENT SYSTEMS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                             04-2925809
(State or other jurisdiction of                       (I.R.S. Employer
 incorporation or organization)                     Identification Number)

                         81 Wyman Street, P.O. Box 9046
                             Waltham, MA 02454-9046
               (Address of Principal Executive Offices) (Zip Code)

              THERMO INSTRUMENT SYSTEMS INC. DEFERRED COMPENSATION
                               PLAN FOR DIRECTORS

           THERMO INSTRUMENT SYSTEMS INC. DIRECTORS STOCK OPTION PLAN

         THERMO INSTRUMENT SYSTEMS INC. EMPLOYEES EQUITY INCENTIVE PLAN

              THERMO INSTRUMENT SYSTEMS INC. EQUITY INCENTIVE PLAN

 THERMO ELECTRON CORPORATION - THERMO INSTRUMENT SYSTEMS INC. NONQUALIFIED STOCK
                                   OPTION PLAN

 THERMO ELECTRON CORPORATION - THERMO INSTRUMENT SYSTEMS INC. 1996 NONQUALIFIED
                                STOCK OPTION PLAN

             THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN

            THERMO INSTRUMENT SYSTEMS INC. INCENTIVE AND NONQUALIFIED
                                STOCK OPTION PLAN


                           (Full Titles of the Plans)
<PAGE>

                          Sandra L. Lambert, Secretary
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                  P.O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                  (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)

                                   Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                  P.O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE


    Title of                                   Proposed
   securities         Amount       Proposed     Maximum            Amount of
to be registered      to be        Maximum     Aggregate         Registration
                    registered     Offering    Offering              Fee
                                   Price Per     Price
                                    Share
  Common Stock,
 $.10 par value     7,548,442     $11.90625   $89,873,637.56       $24,985
    per share         shares         (2)          (2)
                       (1)


     In addition, pursuant to Rule 416(c) under the Securities Act of 1933, this
Registration  Statement  also  covers an  indeterminate  number of shares of the
Registrant's  Common  Stock as may be issuable in  connection  with  adjustments
under the employee  benefit plans described herein to reflect certain changes in
the Registrant's capital structure, including stock dividends or stock splits.

(1)   The shares registered hereunder are divided among the various plans as set
      forth in the following table:

      Name of Plan                                          No. of Shares

      Thermo Instrument Systems Inc. Deferred                     123,502
            Compensation Plan for Directors

      Thermo Instrument Systems Inc. Directors                    175,781
            Stock Option Plan
<PAGE>

      Thermo Instrument Systems Inc. Employees Equity           1,250,000
            Incentive Plan

      Thermo Instrument Systems Inc. Equity Incentive           5,039,062
            Plan

      Thermo Electron Corporation - Thermo Instrument             446,035
            Systems Inc. Nonqualified Stock Option Plan

      Thermo Electron Corporation Directors                       351,562
            Stock Option Plan

      Thermo Instrument Systems Inc. Incentive and                100,000
            Nonqualified Stock Option Plan

      Thermo Electron Corporation - Thermo Instrument              62,500
            Systems Inc. 1996 Nonqualified Stock Option Plan

(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      7,548,442  shares and (2) the  average  of the high and low sales  prices,
      $11.9375 and $11.875,  respectively,  of the Registrant's  Common Stock on
      the  American  Stock  Exchange  on  August  27,  1999 as  reported  in the
      consolidated reporting system.



<PAGE>



                                EXPLANATORY NOTE

      With  respect  to  the  Thermo  Instrument   Systems  Inc.  Incentive  and
Nonqualified  Stock  Option  Plan (the "THI ISO Plan")  and the Thermo  Electron
Corporation / Thermo Instrument Systems Inc. Nonqualified Stock Option Plan (the
"TMO/THI  Plan"),  this  Registration  Statement  is filed  pursuant  to General
Instruction E of Form S-8 in order to register additional securities of the same
class as other  securities for which  registration  statements  relating to such
employee benefit plans are effective.

      On June 10, 1987, the Company filed a  registration  statement on Form S-8
(Reg.  No.  33-14974) to register  100,000  shares of Common Stock  reserved for
issuance  under the TMO/THI Plan.  Giving effect to increases  under the TMO/THI
Plan as a result of stock splits,  the addition of this  Registration  Statement
brings the total number of shares registered for issuance under the TMO/THI Plan
to 973,378.  In addition,  on June 11, 1987,  the Company  filed a  registration
statement on Form S-8 (Reg. No.  33-14980) to register  600,000 shares of Common
Stock reserved for issuance  under the THI ISO Plan.  Giving effect to increases
under  the THI ISO  Plan as a result  of  stock  splits,  the  addition  of this
Registration Statement brings the total number of shares registered for issuance
under  the THI ISO Plan to  3,515,624.  The  contents  of the  previously  filed
registration  statements  relating to the TMO/THI  Plan and the THI ISO Plan are
incorporated  herein by reference except for the items that are set forth below.
Pursuant to Instruction E, this Registration Statement contains such information
required by Form S-8 that is not otherwise included in the previous registration
statements.



                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the  respective  participants  in the plans  listed on the cover page of this
Registration  Statement  pursuant to Rule 428(b)(1)  under the Securities Act of
1933, as amended (the "Securities Act"). As used in this Registration Statement,
the terms "Registrant" or "Company" refer to Thermo Instrument Systems Inc.


                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:
<PAGE>

            (a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999.

            (b)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on January 8, 1999.

            (c)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on March 9, 1999.

            (d)  The  Company's  Quarterly  Report on Form 10-Q for the  quarter
                 ended April 3, 1999.

            (e)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on May 25, 1999.

            (f)  The  Company's  Quarterly  Report on Form 10-Q for the  quarter
                 ended July 3, 1999.

            (g)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.

      All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

      The  validity of the Common Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time  employee of Thermo  Electron  Corporation  ("Thermo  Electron"),  the
majority  stockholder  of the  Company,  is an officer of the Company and Thermo
Electron, and owns or has the right to acquire 18,815 shares of Common Stock and
348,828  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron.
<PAGE>

Item 6.  Indemnification of Directors and Officers.

      The Delaware  General  Corporation  Law and the Company's  Certificate  of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      Thermo  Electron has an insurance  policy which  insures the directors and
officers of Thermo Electron and its subsidiaries, including the Company, against
certain  liabilities  which might be incurred in connection with the performance
of their duties.

Item 7.  Exemption from Registration Claimed.

      Not Applicable.

Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section 10(a)(3)
                           of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;
<PAGE>

                  (iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.

            (2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.

      (b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the opinion of the  Commission  such
indemnification  is  against  public  policy  as  expressed  in the  Act and is,
therefore,  unenforceable. In the event that a claim for indemnification against
such liabilities  (other than the payment by the Registrant of expenses incurred
or paid by a director,  officer or  controlling  person of the Registrant in the
successful  defense of any  action,  suit or  proceeding)  is  asserted  by such
director,  officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been  settled by  controlling  precedent,  submit to a court of  appropriate
jurisdiction the question whether such  indemnification  by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.



<PAGE>


                                   SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Waltham, Commonwealth of Massachusetts,  on this 1st
day of September, 1999.

                                        THERMO INSTRUMENT SYSTEMS INC.


                                        By:  /s/ Earl R. Lewis
                                             ----------------------------------
                                             Earl R. Lewis
                                             President and Chief Executive
                                             Officer


                                POWER OF ATTORNEY

      Each of the  undersigned  Directors  and  Officers  of  Thermo  Instrument
Systems Inc. hereby appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.
Apicerno,  Seth H.  Hoogasian and Sandra L. Lambert,  and each of them, his true
and lawful  attorneys-in-fact  and agents, with full power of substitution,  for
him and in his name, place and stead, in any and all capacities, to sign any and
all  amendments  (including  post-effective  amendments)  to  this  Registration
Statement, and to file the same, with all exhibits thereto, and all documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents, and each of them, full power and authority to
do and perform each and every act and thing  requisite  and necessary to be done
in and about the  premises,  as fully to all intents and purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

Signature                     Title                            Date
---------                     -----                            ----


/s/ Earl R. Lewis             President and Chief           September 1, 1999
-------------------------       Executive Officer
Earl R. Lewis

<PAGE>

/s/ Theo Melas-Kyriazi        Chief Financial Officer       September 1, 1999
-------------------------
Theo Melas-Kyriazi


/s/ Paul F. Kelleher          Chief Accounting Officer      September 1, 1999
-------------------------
Paul F. Kelleher


/s/ Arvin H. Smith            Chairman of the Board         September 1, 1999
-------------------------
Arvin H. Smith

/s/ Frank Borman              Director                      September 1, 1999
-------------------------
Frank Borman


/s/ George N. Hatsopoulos     Director                      September 1, 1999
-------------------------
George N. Hatsopoulos


/s/ John N. Hatsopoulos       Director                      September 1, 1999
-------------------------
John N. Hatsopoulos


/s/ Polyvios C. Vintiadis     Director                      September 1, 1999
-------------------------
Polyvios C. Vintiadis


<PAGE>


EXHIBIT INDEX


Exhibit
Number            Description


5                 Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).



<PAGE>





                                                                     EXHIBIT 5



                         Thermo Instrument Systems Inc.
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046




                                        September 1, 1999

Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, Massachusetts 02454-9046

Re:   Registration Statement on Form S-8
      Relating to 7,548,442 Shares of the Common Stock,
      $.10 par value, of Thermo Instrument Systems Inc.

Dear Sirs:

      I am  General  Counsel  to Thermo  Instrument  Systems  Inc.,  a  Delaware
corporation  (the  "Company"),  and have acted as counsel in connection with the
registration  under the  Securities  Act of 1933,  as amended,  on Form S-8 (the
"Registration  Statement"),  of 7,548,442  shares of the Company's Common Stock,
$.10 par  value per share  (the  "Shares")  subject  to the  following  employee
benefit  plans  (each,  a  "Plan"):  Thermo  Instrument  Systems  Inc.  Deferred
Compensation Plan for Directors,  Thermo Instrument Systems Inc. Directors Stock
Option Plan,  Thermo  Instrument  Systems Inc.  Employees Equity Incentive Plan,
Thermo Instrument  Systems Inc. Equity Incentive Plan, Thermo Instrument Systems
Inc.  Incentive and Nonqualified  Stock Option Plan, Thermo  Instrument  Systems
Inc. / Thermo Electron  Corporation 1996 Nonqualified  Stock Option Plan, Thermo
Electron Corporation / Thermo Instrument Systems Inc.  Nonqualified Stock Option
Plan and Thermo Electron Corporation Directors Stock Option Plan.

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.


<PAGE>

      Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a corporation  duly organized,  validly  existing and in
corporate good standing under the laws of the State of Delaware.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the applicable Plan, will be validly issued, fully paid and nonassessable.

     I  hereby  consent  to the  filing  of this  opinion  as  Exhibit  5 to the
Registration Statement.

                                        Very truly yours,



                                        /s/ Seth H. Hoogasian
                                        -----------------------------
                                        Seth H. Hoogasian
                                        General Counsel


<PAGE>


                                                                    Exhibit 23.1





            Consent of Independent Public Accountants
            -----------------------------------------


     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  registration  statement  on Form S-8 of our report dated
February 16, 1999 (except with respect to the matter discussed in Note 18, as to
which the date is March 1, 1999),  included in Thermo Instrument  Systems Inc.'s
Annual  Report on Form  10-K for the year  ended  January  2,  1999,  and to all
references to our Firm included in this registration statement.


                                                Arthur Andersen LLP



Boston, Massachusetts
August 27, 1999
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 24399}]
{"company-data": {"conformed-name": "THERMO INSTRUMENT SYSTEMS INC", "cik": "0000795986", "assigned-sic": "3829", "irs-number": "042925809", "state-of-incorporation": "DE", "fiscal-year-end": "0102"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-86357", "film-number": "99704716"}, "business-address": {"street1": "860 WEST AIRPORT FREEWAY", "street2": "SUITE 301", "city": "HURST", "state": "TX", "zip": "76054", "phone": "8174856663"}, "mail-address": {"street1": "860 WEST AIRPORT FREEWAY", "street2": "SUITE 301", "city": "HURST", "state": "TX", "zip": "76054"}}
As filed with the Securities and Exchange Commission on September 1, 1999.
                                   Registration No. 333-
         -------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                         THERMO INSTRUMENT SYSTEMS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                             04-2925809
(State or other jurisdiction of                       (I.R.S. Employer
 incorporation or organization)                     Identification Number)

                         81 Wyman Street, P.O. Box 9046
                             Waltham, MA 02454-9046
               (Address of Principal Executive Offices) (Zip Code)

              THERMO INSTRUMENT SYSTEMS INC. DEFERRED COMPENSATION
                               PLAN FOR DIRECTORS

           THERMO INSTRUMENT SYSTEMS INC. DIRECTORS STOCK OPTION PLAN

         THERMO INSTRUMENT SYSTEMS INC. EMPLOYEES EQUITY INCENTIVE PLAN

              THERMO INSTRUMENT SYSTEMS INC. EQUITY INCENTIVE PLAN

 THERMO ELECTRON CORPORATION - THERMO INSTRUMENT SYSTEMS INC. NONQUALIFIED STOCK
                                   OPTION PLAN

 THERMO ELECTRON CORPORATION - THERMO INSTRUMENT SYSTEMS INC. 1996 NONQUALIFIED
                                STOCK OPTION PLAN

             THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN

            THERMO INSTRUMENT SYSTEMS INC. INCENTIVE AND NONQUALIFIED
                                STOCK OPTION PLAN

                           (Full Titles of the Plans)
Sandra L. Lambert, Secretary
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                  P.O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                  (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)

                                   Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                  P.O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE

    Title of                                   Proposed
   securities         Amount       Proposed     Maximum            Amount of
to be registered      to be        Maximum     Aggregate         Registration
                    registered     Offering    Offering              Fee
                                   Price Per     Price
                                    Share
  Common Stock,
 $.10 par value     7,548,442     $11.90625   $89,873,637.56       $24,985
    per share         shares         (2)          (2)
                       (1)

     In addition, pursuant to Rule 416(c) under the Securities Act of 1933, this
Registration  Statement  also  covers an  indeterminate  number of shares of the
Registrant's  Common  Stock as may be issuable in  connection  with  adjustments
under the employee  benefit plans described herein to reflect certain changes in
the Registrant's capital structure, including stock dividends or stock splits.

(1)   The shares registered hereunder are divided among the various plans as set
      forth in the following table:

      Name of Plan                                          No. of Shares

      Thermo Instrument Systems Inc. Deferred                     123,502
            Compensation Plan for Directors

      Thermo Instrument Systems Inc. Directors                    175,781
            Stock Option Plan
Thermo Instrument Systems Inc. Employees Equity           1,250,000
            Incentive Plan

      Thermo Instrument Systems Inc. Equity Incentive           5,039,062
            Plan

      Thermo Electron Corporation - Thermo Instrument             446,035
            Systems Inc. Nonqualified Stock Option Plan

      Thermo Electron Corporation Directors                       351,562
            Stock Option Plan

      Thermo Instrument Systems Inc. Incentive and                100,000
            Nonqualified Stock Option Plan

      Thermo Electron Corporation - Thermo Instrument              62,500
            Systems Inc. 1996 Nonqualified Stock Option Plan

(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      7,548,442  shares and (2) the  average  of the high and low sales  prices,
      $11.9375 and $11.875,  respectively,  of the Registrant's  Common Stock on
      the  American  Stock  Exchange  on  August  27,  1999 as  reported  in the
      consolidated reporting system.
EXPLANATORY NOTE

      With  respect  to  the  Thermo  Instrument   Systems  Inc.  Incentive  and
Nonqualified  Stock  Option  Plan (the "THI ISO Plan")  and the Thermo  Electron
Corporation / Thermo Instrument Systems Inc. Nonqualified Stock Option Plan (the
"TMO/THI  Plan"),  this  Registration  Statement  is filed  pursuant  to General
Instruction E of Form S-8 in order to register additional securities of the same
class as other  securities for which  registration  statements  relating to such
employee benefit plans are effective.

      On June 10, 1987, the Company filed a  registration  statement on Form S-8
(Reg.  No.  33-14974) to register  100,000  shares of Common Stock  reserved for
issuance  under the TMO/THI Plan.  Giving effect to increases  under the TMO/THI
Plan as a result of stock splits,  the addition of this  Registration  Statement
brings the total number of shares registered for issuance under the TMO/THI Plan
to 973,378.  In addition,  on June 11, 1987,  the Company  filed a  registration
statement on Form S-8 (Reg. No.  33-14980) to register  600,000 shares of Common
Stock reserved for issuance  under the THI ISO Plan.  Giving effect to increases
under  the THI ISO  Plan as a result  of  stock  splits,  the  addition  of this
Registration Statement brings the total number of shares registered for issuance
under  the THI ISO Plan to  3,515,624.  The  contents  of the  previously  filed
registration  statements  relating to the TMO/THI  Plan and the THI ISO Plan are
incorporated  herein by reference except for the items that are set forth below.
Pursuant to Instruction E, this Registration Statement contains such information
required by Form S-8 that is not otherwise included in the previous registration
statements.

                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the  respective  participants  in the plans  listed on the cover page of this
Registration  Statement  pursuant to Rule 428(b)(1)  under the Securities Act of
1933, as amended (the "Securities Act"). As used in this Registration Statement,
the terms "Registrant" or "Company" refer to Thermo Instrument Systems Inc.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:
(a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999.

            (b)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on January 8, 1999.

            (c)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on March 9, 1999.

            (d)  The  Company's  Quarterly  Report on Form 10-Q for the  quarter
                 ended April 3, 1999.

            (e)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on May 25, 1999.

            (f)  The  Company's  Quarterly  Report on Form 10-Q for the  quarter
                 ended July 3, 1999.

            (g)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.

      All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

      The  validity of the Common Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time  employee of Thermo  Electron  Corporation  ("Thermo  Electron"),  the
majority  stockholder  of the  Company,  is an officer of the Company and Thermo
Electron, and owns or has the right to acquire 18,815 shares of Common Stock and
348,828  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron.
Item 6.  Indemnification of Directors and Officers.

      The Delaware  General  Corporation  Law and the Company's  Certificate  of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      Thermo  Electron has an insurance  policy which  insures the directors and
officers of Thermo Electron and its subsidiaries, including the Company, against
certain  liabilities  which might be incurred in connection with the performance
of their duties.

Item 7.  Exemption from Registration Claimed.

      Not Applicable.

Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section 10(a)(3)
                           of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;
(iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.

            (2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.

      (b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the opinion of the  Commission  such
indemnification  is  against  public  policy  as  expressed  in the  Act and is,
therefore,  unenforceable. In the event that a claim for indemnification against
such liabilities  (other than the payment by the Registrant of expenses incurred
or paid by a director,  officer or  controlling  person of the Registrant in the
successful  defense of any  action,  suit or  proceeding)  is  asserted  by such
director,  officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been  settled by  controlling  precedent,  submit to a court of  appropriate
jurisdiction the question whether such  indemnification  by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.
SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Waltham, Commonwealth of Massachusetts,  on this 1st
day of September, 1999.

                                        THERMO INSTRUMENT SYSTEMS INC.

                                        By:  /s/ Earl R. Lewis
                                             ----------------------------------
                                             Earl R. Lewis
                                             President and Chief Executive
                                             Officer

                                POWER OF ATTORNEY

      Each of the  undersigned  Directors  and  Officers  of  Thermo  Instrument
Systems Inc. hereby appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.
Apicerno,  Seth H.  Hoogasian and Sandra L. Lambert,  and each of them, his true
and lawful  attorneys-in-fact  and agents, with full power of substitution,  for
him and in his name, place and stead, in any and all capacities, to sign any and
all  amendments  (including  post-effective  amendments)  to  this  Registration
Statement, and to file the same, with all exhibits thereto, and all documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents, and each of them, full power and authority to
do and perform each and every act and thing  requisite  and necessary to be done
in and about the  premises,  as fully to all intents and purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

Signature                     Title                            Date
---------                     -----                            ----

/s/ Earl R. Lewis             President and Chief           September 1, 1999
-------------------------       Executive Officer
Earl R. Lewis
/s/ Theo Melas-Kyriazi        Chief Financial Officer       September 1, 1999
-------------------------
Theo Melas-Kyriazi

/s/ Paul F. Kelleher          Chief Accounting Officer      September 1, 1999
-------------------------
Paul F. Kelleher

/s/ Arvin H. Smith            Chairman of the Board         September 1, 1999
-------------------------
Arvin H. Smith

/s/ Frank Borman              Director                      September 1, 1999
-------------------------
Frank Borman

/s/ George N. Hatsopoulos     Director                      September 1, 1999
-------------------------
George N. Hatsopoulos

/s/ John N. Hatsopoulos       Director                      September 1, 1999
-------------------------
John N. Hatsopoulos

/s/ Polyvios C. Vintiadis     Director                      September 1, 1999
-------------------------
Polyvios C. Vintiadis
EXHIBIT INDEX

Exhibit
Number            Description

5                 Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).
EXHIBIT 5

                         Thermo Instrument Systems Inc.
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046

                                        September 1, 1999

Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, Massachusetts 02454-9046

Re:   Registration Statement on Form S-8
      Relating to 7,548,442 Shares of the Common Stock,
      $.10 par value, of Thermo Instrument Systems Inc.

Dear Sirs:

      I am  General  Counsel  to Thermo  Instrument  Systems  Inc.,  a  Delaware
corporation  (the  "Company"),  and have acted as counsel in connection with the
registration  under the  Securities  Act of 1933,  as amended,  on Form S-8 (the
"Registration  Statement"),  of 7,548,442  shares of the Company's Common Stock,
$.10 par  value per share  (the  "Shares")  subject  to the  following  employee
benefit  plans  (each,  a  "Plan"):  Thermo  Instrument  Systems  Inc.  Deferred
Compensation Plan for Directors,  Thermo Instrument Systems Inc. Directors Stock
Option Plan,  Thermo  Instrument  Systems Inc.  Employees Equity Incentive Plan,
Thermo Instrument  Systems Inc. Equity Incentive Plan, Thermo Instrument Systems
Inc.  Incentive and Nonqualified  Stock Option Plan, Thermo  Instrument  Systems
Inc. / Thermo Electron  Corporation 1996 Nonqualified  Stock Option Plan, Thermo
Electron Corporation / Thermo Instrument Systems Inc.  Nonqualified Stock Option
Plan and Thermo Electron Corporation Directors Stock Option Plan.

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.
Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a corporation  duly organized,  validly  existing and in
corporate good standing under the laws of the State of Delaware.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the applicable Plan, will be validly issued, fully paid and nonassessable.

     I  hereby  consent  to the  filing  of this  opinion  as  Exhibit  5 to the
Registration Statement.

                                        Very truly yours,

                                        /s/ Seth H. Hoogasian
                                        -----------------------------
                                        Seth H. Hoogasian
                                        General Counsel
Exhibit 23.1

            Consent of Independent Public Accountants
            -----------------------------------------

     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  registration  statement  on Form S-8 of our report dated
February 16, 1999 (except with respect to the matter discussed in Note 18, as to
which the date is March 1, 1999),  included in Thermo Instrument  Systems Inc.'s
Annual  Report on Form  10-K for the year  ended  January  2,  1999,  and to all
references to our Firm included in this registration statement.

                                                Arthur Andersen LLP

Boston, Massachusetts
August 27, 1999