<PAGE>   1
       As filed with the Securities and Exchange Commission on May 7, 1999
                                                      Registration No. 333-_____
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549


                                    FORM S-8


                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933


                              MKS INSTRUMENTS, INC.
             (Exact Name of Registrant as Specified in Its Charter)


                                  MASSACHUSETTS
         (State or Other Jurisdiction of Incorporation or Organization)


                                   04-2277512
                      (I.R.S. Employer Identification No.)


SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS                               01810
(Address of Principal Executive Offices)                             (Zip Code)


                         1997 DIRECTOR STOCK OPTION PLAN
                            (Full Title of the Plan)


                              MARK G. BORDEN, ESQ.
                                HALE AND DORR LLP
                                 60 STATE STREET
                           BOSTON, MASSACHUSETTS 02109
                     (Name and Address of Agent for Service)


                                 (617) 526-6000
          (Telephone Number, Including Area Code, of Agent For Service)


                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
================================================================================
                                   PROPOSED        PROPOSED                     
 TITLE OF                           MAXIMUM        MAXIMUM                      
SECURITIES          AMOUNT         OFFERING       AGGREGATE           AMOUNT OF
   TO BE            TO BE          PRICE PER       OFFERING         REGISTRATION
REGISTERED        REGISTERED         SHARE          PRICE                FEE
--------------------------------------------------------------------------------
<S>                <C>             <C>           <C>                   <C>   
Common Stock,      300,000         $12.63(1)     $3,789,000(1)         $1,053
no par value
================================================================================
</TABLE>

(1)  Estimated solely for the purpose of calculating the registration fee, and
     based upon the average of the high and low prices of the Common Stock on
     the Nasdaq National Market on May 5, 1999 in accordance with Rules 457(c)
     and 457(h) of the Securities Act of 1933, as amended.

================================================================================




<PAGE>   2





PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

         The information required by Part I is included in documents sent or
given to participants in the Registrant's 1997 Director Stock Option Plan
pursuant to Rule 428(b)(1) of the Securities Act of 1933, as amended (the
"Securities Act").


PART II. INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 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 filed with the
Commission, are incorporated in this Registration Statement by reference:

                  (1)      The Registrant's latest prospectus filed pursuant to
         Rule 424(b) under the Securities Act that contains audited financial
         statements for the Registrant's latest fiscal year for which such
         statements have been filed.

                  (2)      All other reports filed pursuant to Section 13(a) or
         15(d) of the Exchange Act since the end of the fiscal year covered by
         the prospectus referred to in (1) above.

                  (3)      The description of the common stock of the
         Registrant, no par value per share (the "Common Stock"), contained in
         the Registration Statement on Form 8-A filed by the Registrant with the
         Commission on March 2, 1999.

         All documents subsequently filed by the Registrant pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a
post-effective amendment which indicates that all shares of Common Stock offered
hereby have been sold or which deregisters all shares of Common Stock then
remaining unsold, shall be deemed to be incorporated by reference herein and to
be part hereof from the date of the filing of such documents.





                                      II-1



<PAGE>   3



Item 4.  DESCRIPTION OF SECURITIES

         Not applicable.

Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL

         Not applicable.

Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

         Section 67 of Chapter 156B of the Massachusetts General Laws provides
that a corporation may indemnify its directors and officers to the extent
specified in or authorized by (1) the articles of organization; (2) a by-law
adopted by the stockholders; or (3) a vote adopted by the holders of a majority
of the shares of stock entitled to vote on the election of directors. In all
instances, the extent to which a corporation provides indemnification to its
directors and officers under Section 67 is optional. In its Restated Articles of
Organization (the "Articles of Organization"), the Registrant has elected to
commit to provide indemnification to its directors and officers in specified
circumstances. Generally, Article 6 of the Registrant's Articles of Organization
provides that the Registrant shall indemnify directors and officers of the
Registrant against liabilities and expenses arising out of legal proceedings
brought against them by reason of their status as directors or officers or by
reason of their agreeing to serve, at the request of the Registrant, as a
director or officer with another organization. Under this provision, a director
or officer of the Registrant shall be indemnified by the Registrant for all
costs and expenses (including attorneys' fees), judgments, liabilities and
amounts paid in settlement of such proceedings, even if he is not successful on
the merits, if he acted in good faith in the reasonable belief that his action
was in the best interests of the Registrant. The Board of Directors may
authorize advancing litigation expenses to a director or officer at his request
upon receipt of an undertaking by any such director or officer to repay such
expenses if it is ultimately determined that he is not entitled to
indemnification for such expenses.

         Article 6 of the Registrant's Articles of Organization eliminates the
personal liability of the Registrant's directors to the Registrant or its
stockholders for monetary damages for breach of a director's fiduciary duty,
except to the extent Chapter 156B of the Massachusetts General Laws prohibits
the elimination or limitation of such liability.

         The Registrant has obtained directors and officers liability insurance
for the benefit of its directors and certain of its officers.



Item 7.  EXEMPTION FROM REGISTRATION CLAIMED




                                      II-2



<PAGE>   4



         Not applicable.


Item 8.  EXHIBITS

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


Item 9.  UNDERTAKINGS

         1.       The 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 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;

                  (2)      That, for the purpose of determining any liability
         under the Securities Act, 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.

         2.       The undersigned Registrant hereby undertakes that, for
purposes of determining any liability under the Securities Act, each filing of
the Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) 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 in the initial
bona fide offering thereof.

         3.       Insofar as indemnification for liabilities arising under the
Securities Act 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 Securities 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




                                      II-3



<PAGE>   5





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 Securities Act and will be governed by
the final adjudication of such issue.



                                      II-4



<PAGE>   6



                                   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 Andover, Commonwealth of Massachusetts, on this 7th
day of May, 1999.




                                   MKS INSTRUMENTS, INC.



                                   By: /s/ John R. Bertucci
                                       -------------------------------------
                                       John R. Bertucci
                                       Chairman of the Board,
                                       President and Chief Executive Officer





                                      II-5



<PAGE>   7



                                POWER OF ATTORNEY

         We, the undersigned officers and directors of MKS Instruments, Inc.,
hereby severally constitute John R. Bertucci, Ronald C. Weigner and Mark G.
Borden, and each of them singly, our true and lawful attorneys with full power
to them, and each of them singly, to sign for us and in our names in the
capacities indicated below, the Registration Statement on Form S-8 filed
herewith and any and all subsequent amendments to said Registration Statement,
and generally to do all such things in our names and behalf in our capacities as
officers and directors to enable MKS Instruments, Inc. to comply with all
requirements of the Securities and Exchange Commission, hereby ratifying and
confirming our signatures as they may be signed by said attorneys, or any of
them, to said Registration Statement and any and all amendments thereto.

         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 date indicated.



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


/s/ John R. Bertucci              Chairman of the Board,           May 7, 1999
----------------------------      President and Chief
John R. Bertucci                  Executive Officer
                                  (Principal Executive
                                  Officer)


/s/ Ronald C. Weigner             Vice President and               May 7, 1999
----------------------------      Chief Financial Officer
Ronald C. Weigner



/s/ Richard S. Chute              Director                         May 7, 1999
----------------------------
Richard S. Chute



/s/ Owen W. Robbins               Director                         May 7, 1999
----------------------------
Owen W. Robbins



/s/ Robert J. Therrien            Director                         May 7, 1999
----------------------------
Robert J. Therrien



/s/ Louis P. Valente              Director                         May 7, 1999
----------------------------
Louis P. Valente





                                      II-6



<PAGE>   8





                                  EXHIBIT INDEX



Exhibit
Number                            Description
-------                           -----------

4(1)      Specimen Certificate for shares of Common Stock, no par value per
          share, of the Registrant

5         Opinion of Hale and Dorr LLP

23.1      Consent of Hale and Dorr LLP (included in Exhibit 5)

23.2      Consent of PricewaterhouseCoopers LLP

24        Power of Attorney (included on the signature page of this Registration
          Statement)







----------
1    Incorporated herein by reference to the Registrant's Registration Statement
     on Form S-1 (Commission File No. 333-71363).




                                      II-7
[{"type": "S-8", "sequence": "1", "description": "MKS INSTRUMENTS, INC.", "secsgml_size_bytes": 14581}]
{"company-data": {"conformed-name": "MKS INSTRUMENTS INC", "cik": "0001049502", "assigned-sic": "3823", "irs-number": "042277512", "state-of-incorporation": "MA", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-78073", "film-number": "99614382"}, "business-address": {"street1": "512 SHATTUCK RD", "city": "ANDOVER", "state": "MA", "zip": "01810", "phone": "9789752350"}, "mail-address": {"street1": "512 SHATTUCK ROAD", "city": "ANDOVER", "state": "MA", "zip": "01810"}}
1
       As filed with the Securities and Exchange Commission on May 7, 1999
                                                      Registration No. 333-_____
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8

                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

                              MKS INSTRUMENTS, INC.
             (Exact Name of Registrant as Specified in Its Charter)

                                  MASSACHUSETTS
         (State or Other Jurisdiction of Incorporation or Organization)

                                   04-2277512
                      (I.R.S. Employer Identification No.)

SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS                               01810
(Address of Principal Executive Offices)                             (Zip Code)

                         1997 DIRECTOR STOCK OPTION PLAN
                            (Full Title of the Plan)

                              MARK G. BORDEN, ESQ.
                                HALE AND DORR LLP
                                 60 STATE STREET
                           BOSTON, MASSACHUSETTS 02109
                     (Name and Address of Agent for Service)

                                 (617) 526-6000
          (Telephone Number, Including Area Code, of Agent For Service)

                         CALCULATION OF REGISTRATION FEE
================================================================================
                                   PROPOSED        PROPOSED                     
 TITLE OF                           MAXIMUM        MAXIMUM                      
SECURITIES          AMOUNT         OFFERING       AGGREGATE           AMOUNT OF
   TO BE            TO BE          PRICE PER       OFFERING         REGISTRATION
REGISTERED        REGISTERED         SHARE          PRICE                FEE
--------------------------------------------------------------------------------
Common Stock,      300,000         $12.63(1)     $3,789,000(1)         $1,053
no par value
================================================================================
(1)  Estimated solely for the purpose of calculating the registration fee, and
     based upon the average of the high and low prices of the Common Stock on
     the Nasdaq National Market on May 5, 1999 in accordance with Rules 457(c)
     and 457(h) of the Securities Act of 1933, as amended.

================================================================================
2

PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

         The information required by Part I is included in documents sent or
given to participants in the Registrant's 1997 Director Stock Option Plan
pursuant to Rule 428(b)(1) of the Securities Act of 1933, as amended (the
"Securities Act").

PART II. INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 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 filed with the
Commission, are incorporated in this Registration Statement by reference:

                  (1)      The Registrant's latest prospectus filed pursuant to
         Rule 424(b) under the Securities Act that contains audited financial
         statements for the Registrant's latest fiscal year for which such
         statements have been filed.

                  (2)      All other reports filed pursuant to Section 13(a) or
         15(d) of the Exchange Act since the end of the fiscal year covered by
         the prospectus referred to in (1) above.

                  (3)      The description of the common stock of the
         Registrant, no par value per share (the "Common Stock"), contained in
         the Registration Statement on Form 8-A filed by the Registrant with the
         Commission on March 2, 1999.

         All documents subsequently filed by the Registrant pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a
post-effective amendment which indicates that all shares of Common Stock offered
hereby have been sold or which deregisters all shares of Common Stock then
remaining unsold, shall be deemed to be incorporated by reference herein and to
be part hereof from the date of the filing of such documents.

                                      II-1
3

Item 4.  DESCRIPTION OF SECURITIES

         Not applicable.

Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL

         Not applicable.

Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

         Section 67 of Chapter 156B of the Massachusetts General Laws provides
that a corporation may indemnify its directors and officers to the extent
specified in or authorized by (1) the articles of organization; (2) a by-law
adopted by the stockholders; or (3) a vote adopted by the holders of a majority
of the shares of stock entitled to vote on the election of directors. In all
instances, the extent to which a corporation provides indemnification to its
directors and officers under Section 67 is optional. In its Restated Articles of
Organization (the "Articles of Organization"), the Registrant has elected to
commit to provide indemnification to its directors and officers in specified
circumstances. Generally, Article 6 of the Registrant's Articles of Organization
provides that the Registrant shall indemnify directors and officers of the
Registrant against liabilities and expenses arising out of legal proceedings
brought against them by reason of their status as directors or officers or by
reason of their agreeing to serve, at the request of the Registrant, as a
director or officer with another organization. Under this provision, a director
or officer of the Registrant shall be indemnified by the Registrant for all
costs and expenses (including attorneys' fees), judgments, liabilities and
amounts paid in settlement of such proceedings, even if he is not successful on
the merits, if he acted in good faith in the reasonable belief that his action
was in the best interests of the Registrant. The Board of Directors may
authorize advancing litigation expenses to a director or officer at his request
upon receipt of an undertaking by any such director or officer to repay such
expenses if it is ultimately determined that he is not entitled to
indemnification for such expenses.

         Article 6 of the Registrant's Articles of Organization eliminates the
personal liability of the Registrant's directors to the Registrant or its
stockholders for monetary damages for breach of a director's fiduciary duty,
except to the extent Chapter 156B of the Massachusetts General Laws prohibits
the elimination or limitation of such liability.

         The Registrant has obtained directors and officers liability insurance
for the benefit of its directors and certain of its officers.

Item 7.  EXEMPTION FROM REGISTRATION CLAIMED

                                      II-2
4

         Not applicable.

Item 8.  EXHIBITS

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

Item 9.  UNDERTAKINGS

         1.       The 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 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;

                  (2)      That, for the purpose of determining any liability
         under the Securities Act, 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.

         2.       The undersigned Registrant hereby undertakes that, for
purposes of determining any liability under the Securities Act, each filing of
the Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) 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 in the initial
bona fide offering thereof.

         3.       Insofar as indemnification for liabilities arising under the
Securities Act 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 Securities 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

                                      II-3
5

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 Securities Act and will be governed by
the final adjudication of such issue.

                                      II-4
6

                                   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 Andover, Commonwealth of Massachusetts, on this 7th
day of May, 1999.

                                   MKS INSTRUMENTS, INC.

                                   By: /s/ John R. Bertucci
                                       -------------------------------------
                                       John R. Bertucci
                                       Chairman of the Board,
                                       President and Chief Executive Officer

                                      II-5
7

                                POWER OF ATTORNEY

         We, the undersigned officers and directors of MKS Instruments, Inc.,
hereby severally constitute John R. Bertucci, Ronald C. Weigner and Mark G.
Borden, and each of them singly, our true and lawful attorneys with full power
to them, and each of them singly, to sign for us and in our names in the
capacities indicated below, the Registration Statement on Form S-8 filed
herewith and any and all subsequent amendments to said Registration Statement,
and generally to do all such things in our names and behalf in our capacities as
officers and directors to enable MKS Instruments, Inc. to comply with all
requirements of the Securities and Exchange Commission, hereby ratifying and
confirming our signatures as they may be signed by said attorneys, or any of
them, to said Registration Statement and any and all amendments thereto.

         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 date indicated.

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

/s/ John R. Bertucci              Chairman of the Board,           May 7, 1999
----------------------------      President and Chief
John R. Bertucci                  Executive Officer
                                  (Principal Executive
                                  Officer)

/s/ Ronald C. Weigner             Vice President and               May 7, 1999
----------------------------      Chief Financial Officer
Ronald C. Weigner

/s/ Richard S. Chute              Director                         May 7, 1999
----------------------------
Richard S. Chute

/s/ Owen W. Robbins               Director                         May 7, 1999
----------------------------
Owen W. Robbins

/s/ Robert J. Therrien            Director                         May 7, 1999
----------------------------
Robert J. Therrien

/s/ Louis P. Valente              Director                         May 7, 1999
----------------------------
Louis P. Valente

                                      II-6
8

                                  EXHIBIT INDEX

Exhibit
Number                            Description
-------                           -----------

4(1)      Specimen Certificate for shares of Common Stock, no par value per
          share, of the Registrant

5         Opinion of Hale and Dorr LLP

23.1      Consent of Hale and Dorr LLP (included in Exhibit 5)

23.2      Consent of PricewaterhouseCoopers LLP

24        Power of Attorney (included on the signature page of this Registration
          Statement)

----------
1    Incorporated herein by reference to the Registrant's Registration Statement
     on Form S-1 (Commission File No. 333-71363).

                                      II-7