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):

                                  July 21, 1998

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


Delaware                      1-9786                        04-2925809
(State or other             (Commission                (I.R.S. Employer
jurisdiction of               File Number)             Identification Number) 
incorporation or
organization)


860 West Airport Freeway, Suite 301
Hurst, Texas                                                       76054
(Address of principal executive offices)                         (Zip Code)


                                (817) 485-6663
                         (Registrant's telephone number
                              including area code)












Item 5.     Other Events

      On July 21, 1998, Thermo Instrument  Systems Inc. (the "Company") issued a
press release,  attached hereto as Exhibit 99, regarding its expected  operating
income for the quarter ended July 4, 1998.

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

            99    Press Release of the Company, dated July 21,
                  1998











                                    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 22nd day of July, 1998.



                                    THERMO INSTRUMENT SYSTEMS INC.



                                    By: /s/ Sandra L. Lambert
                                          Sandra L. Lambert
                                          Secretary









EXHIBIT 99

               THERMO INSTRUMENT SYSTEMS EXPECTS SECOND QUARTER
               OPERATING INCOME TO BE APPROXIMATELY $55 MILLION

BEDFORD,  Texas,  July 21,  1998 -- Thermo  Instrument  Systems  Inc.  (ASE-THI)
announced today that it expects  operating income for the second quarter,  ended
July 4, 1998, to be approximately $55 million. Lower sales as a result of orders
received late in the quarter at the company's  ThermoQuest  (ASE-TMQ) subsidiary
were the primary cause of the  lower-than-expected  operating  income  estimate.
Thermo Instrument  Systems' diluted earnings per share for the 1998 quarter will
be approximately the same as the 1997 period.

      Thermo Instrument Systems plans to release its complete quarterly earnings
results on August 13, 1998.

     Thermo  Instrument  Systems  Inc.  is  a  leading  provider  of  analytical
instruments used to identify complex chemical compounds, toxic metals, and other
elements in a broad range of liquids and solids.  The company also  develops and
manufactures  instruments used to monitor radioactivity and air pollution;  life
science instruments and consumables; and imaging,  inspection,  measurement, and
control instruments for various industrial processes.  Thermo Instrument Systems
is a public  subsidiary of Thermo  Electron  Corporation.  More  information  is
available on the Internet at http://www.thermo.com/subsid/thi1.html.

      This press  release  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 the  company's  annual  report  on Form  10-K for the year  ended
January  3,  1998.  These  include  uncertainties   relating  to  the  company's
acquisition  and spinout  strategies,  risks  associated  with  competition  and
technological change, intellectual property rights and litigation, dependence on
certain  key  industries  and  international  operations,  possible  changes  in
governmental regulations,  capital spending and government funding policies, and
the potential impact of the year 2000 on processing date-sensitive information.