SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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


                                    FORM 8-K


                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       the Securities Exchange Act of 1934



                                 Date of Report
                       (Date of earliest event reported):

                                December 14, 1999

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


                             THERMOTREX CORPORATION
             (Exact name of Registrant as specified in its charter)


Delaware                        1-10791                          52-1711436
(State or other              (Commission                    (I.R.S. Employer
jurisdiction of              File Number)                Identification Number)
incorporation or
organization)


81 Wyman Street
Waltham, Massachusetts                                                02454
(Address of principal executive offices)                           (Zip Code)


                                (781) 622-1000
                         (Registrant's telephone number
                              including area code)



<PAGE>




This Form 8-K contains forward-looking statements that involve a number of risks
and  uncertainties.  Important factors that could cause actual results to differ
materially from those indicated by such forward-looking statements are set forth
under the  heading  "Forward-looking  Statements"  in Exhibit  13 to  ThermoTrex
Corporation's  Annual  Report on Form 10-K for the year  ended  October 3, 1998.
These  include  risks and  uncertainties  relating  to:  the need for  continued
product development and commercialization of products under development;  market
acceptance;   difficulty  in  retaining  qualified   management  and  personnel;
dependence  on capital  spending  policies;  risks  associated  with  government
contracts; risk of decreases in military weapons spending; government regulation
and need for regulatory approvals;  healthcare reform and uncertainty of patient
reimbursement; intense competition; intellectual property rights, uncertainties,
and litigation;  operating  losses at ThermoLase;  potential for customer claims
and  product  liability;   dependence  upon  significant  relationships;   risks
associated with acquisition strategy;  and the potential impact of the year 2000
on processing date-sensitive information.

Item 5.     Other Events

      On December 17, 1999, the Registrant  issued a press release  stating that
it has entered  into a definitive  agreement  and plan of merger with its parent
corporation, Thermo Electron Corporation ("Thermo Electron"), under which Thermo
Electron would acquire all of the outstanding  common stock,  $.01 par value per
share (the "Common  Stock"),  held by the  stockholders of the Registrant  other
than Thermo Electron and its  affiliates.  The  Registrant's  board of directors
approved  the  merger  agreement  based  on  a  recommendation  of  its  special
committee, which was charged with representing the interests of the Registrant's
public shareholders.

      Under  the  agreement,   each  share  of  Common  Stock  owned  by  public
shareholders  would be  automatically  converted into the right to receive .5503
shares of Thermo  Electron's  common stock,  $1.00 par value per share (the "TMO
Common Stock").

      Any shares of TMO Common  Stock  issued  pursuant to the merger  agreement
would be offered only by means of a prospectus.

       The Registrant expects that the necessary filings with the Securities and
Exchange  Commission (the "SEC") will be made shortly,  and that proxy materials
for a special meeting will be mailed to stockholders of the Registrant  promptly
after completion of SEC review. The completion of this transaction is subject to
certain customary  conditions,  including completion of review by the SEC of the
registration statement regarding the proposed transaction.  A copy of the merger
agreement  is  filed  as  an  exhibit  hereto.   Thermo  Electron,   which  owns
approximately  80% of the  Common  Stock,  intends  to vote all of its shares of
Common  Stock in favor of  approval  of the  merger  agreement  and,  therefore,
stockholder approval is assured. The transaction is expected to close during the
second  calendar  quarter of 2000.  Following  the merger,  the shares of Common
Stock would cease to be publicly traded.

Item 7.     Financial Statements, Pro Forma Financial Information and Exhibits

      (a)    Financial Statements of Business Acquired: not applicable.
<PAGE>

      (b)   Pro Forma Financial Information: not applicable.

      (c)   Exhibits:

                   2.1  Agreement  and Plan of Merger  dated as of December  14,
            1999  by  and  among   Thermo   Electron   Corporation,   ThermoTrex
            Acquisition Corporation and ThermoTrex Corporation.




<PAGE>



                                    SIGNATURE



      Pursuant to the  requirements of the Securities  Exchange Act of 1934, the
Registrant  has duly  caused  this  report  to be  signed  on its  behalf by the
undersigned, thereunto duly authorized, on this 17th day of December, 1999.



                                        THERMOTREX CORPORATION


                                        By:  /s/ Theo Melas-Kyriazi
                                             ----------------------------
                                             Theo Melas-Kyriazi
                                             Chief Financial Officer
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SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       the Securities Exchange Act of 1934

                                 Date of Report
                       (Date of earliest event reported):

                                December 14, 1999

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

                             THERMOTREX CORPORATION
             (Exact name of Registrant as specified in its charter)

Delaware                        1-10791                          52-1711436
(State or other              (Commission                    (I.R.S. Employer
jurisdiction of              File Number)                Identification Number)
incorporation or
organization)

81 Wyman Street
Waltham, Massachusetts                                                02454
(Address of principal executive offices)                           (Zip Code)

                                (781) 622-1000
                         (Registrant's telephone number
                              including area code)
This Form 8-K contains forward-looking statements that involve a number of risks
and  uncertainties.  Important factors that could cause actual results to differ
materially from those indicated by such forward-looking statements are set forth
under the  heading  "Forward-looking  Statements"  in Exhibit  13 to  ThermoTrex
Corporation's  Annual  Report on Form 10-K for the year  ended  October 3, 1998.
These  include  risks and  uncertainties  relating  to:  the need for  continued
product development and commercialization of products under development;  market
acceptance;   difficulty  in  retaining  qualified   management  and  personnel;
dependence  on capital  spending  policies;  risks  associated  with  government
contracts; risk of decreases in military weapons spending; government regulation
and need for regulatory approvals;  healthcare reform and uncertainty of patient
reimbursement; intense competition; intellectual property rights, uncertainties,
and litigation;  operating  losses at ThermoLase;  potential for customer claims
and  product  liability;   dependence  upon  significant  relationships;   risks
associated with acquisition strategy;  and the potential impact of the year 2000
on processing date-sensitive information.

Item 5.     Other Events

      On December 17, 1999, the Registrant  issued a press release  stating that
it has entered  into a definitive  agreement  and plan of merger with its parent
corporation, Thermo Electron Corporation ("Thermo Electron"), under which Thermo
Electron would acquire all of the outstanding  common stock,  $.01 par value per
share (the "Common  Stock"),  held by the  stockholders of the Registrant  other
than Thermo Electron and its  affiliates.  The  Registrant's  board of directors
approved  the  merger  agreement  based  on  a  recommendation  of  its  special
committee, which was charged with representing the interests of the Registrant's
public shareholders.

      Under  the  agreement,   each  share  of  Common  Stock  owned  by  public
shareholders  would be  automatically  converted into the right to receive .5503
shares of Thermo  Electron's  common stock,  $1.00 par value per share (the "TMO
Common Stock").

      Any shares of TMO Common  Stock  issued  pursuant to the merger  agreement
would be offered only by means of a prospectus.

       The Registrant expects that the necessary filings with the Securities and
Exchange  Commission (the "SEC") will be made shortly,  and that proxy materials
for a special meeting will be mailed to stockholders of the Registrant  promptly
after completion of SEC review. The completion of this transaction is subject to
certain customary  conditions,  including completion of review by the SEC of the
registration statement regarding the proposed transaction.  A copy of the merger
agreement  is  filed  as  an  exhibit  hereto.   Thermo  Electron,   which  owns
approximately  80% of the  Common  Stock,  intends  to vote all of its shares of
Common  Stock in favor of  approval  of the  merger  agreement  and,  therefore,
stockholder approval is assured. The transaction is expected to close during the
second  calendar  quarter of 2000.  Following  the merger,  the shares of Common
Stock would cease to be publicly traded.

Item 7.     Financial Statements, Pro Forma Financial Information and Exhibits

      (a)    Financial Statements of Business Acquired: not applicable.
(b)   Pro Forma Financial Information: not applicable.

      (c)   Exhibits:

                   2.1  Agreement  and Plan of Merger  dated as of December  14,
            1999  by  and  among   Thermo   Electron   Corporation,   ThermoTrex
            Acquisition Corporation and ThermoTrex Corporation.
SIGNATURE

      Pursuant to the  requirements of the Securities  Exchange Act of 1934, the
Registrant  has duly  caused  this  report  to be  signed  on its  behalf by the
undersigned, thereunto duly authorized, on this 17th day of December, 1999.

                                        THERMOTREX CORPORATION

                                        By:  /s/ Theo Melas-Kyriazi
                                             ----------------------------
                                             Theo Melas-Kyriazi
                                             Chief Financial Officer