SECURITIES AND EXCHANGE COMMISSION

                              Washington, DC 20549

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

                                    FORM 10-Q

(mark one)

[ X ] Quarterly Report Pursuant to Section 13 or 15(d) of the Securities
      Exchange Act of 1934 for the Quarter Ended July 4, 1998.

[   ] Transition Report Pursuant to Section 13 or 15(d) of the Securities
      Exchange Act of 1934.

                          Commission File Number 1-9786

                         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 No.)

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

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

           Indicate by check mark whether the Registrant (1) has filed all
           reports required to be filed by Section 13 or 15(d) of the Securities
           Exchange Act of 1934 during the preceding 12 months (or for such
           shorter period that the Registrant was required to file such
           reports), and (2) has been subject to such filing requirements for
           the past 90 days. Yes [ X ] No [ ]

           Indicate the number of shares outstanding of each of the issuer's
           classes of Common Stock, as of the latest practicable date.

                Class                    Outstanding at July 31, 1998
     ----------------------------        ----------------------------
     Common Stock, $.10 par value                 122,198,347


                                       
<PAGE>

PART I - FINANCIAL INFORMATION

Item 1 - Financial Statements

                         THERMO INSTRUMENT SYSTEMS INC.

                           Consolidated Balance Sheet
                                   (Unaudited)

                                     Assets


                                                      July 4,   January 3,
(In thousands)                                           1998         1998
--------------------------------------------------------------------------

Current Assets:
  Cash and cash equivalents (includes $560,460
    and $283,995 under repurchase agreement with
    parent company)                                $  730,139   $  468,848
  Available-for-sale investments, at quoted
    market value (amortized cost of $8,228
    and $8,287)                                         9,014        8,328
  Accounts receivable, less allowances of
    $21,406 and $22,786                               341,204      364,075
  Unbilled contract costs and fees                      8,646        9,191
  Inventories:
    Raw materials and supplies                        128,500      118,611
    Work in process                                    56,691       52,870
    Finished goods                                     99,363       93,238
  Prepaid expenses                                     23,368       19,292
  Prepaid income taxes                                 57,087       54,915
                                                   ----------   ----------

                                                    1,454,012    1,189,368
                                                   ----------   ----------

Property, Plant, and Equipment, at Cost               322,939      317,605
  Less: Accumulated depreciation and
        amortization                                  112,537       97,666
                                                   ----------   ----------

                                                      210,402      219,939
                                                   ----------   ----------

Other Assets                                           52,114       45,477
                                                   ----------   ----------

Cost in Excess of Net Assets of Acquired
  Companies (Note 6)                                  901,009      896,369
                                                   ----------   ----------

                                                   $2,617,537   $2,351,153
                                                   ==========   ==========

                                       2
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.


                     Consolidated Balance Sheet (continued)
                                   (Unaudited)

                  Liabilities and Shareholders' Investment

                                                      July 4,   January 3,
(In thousands except share amounts)                      1998         1998
--------------------------------------------------------------------------

Current Liabilities:
  Notes payable and current maturities of
    long-term obligations (includes $65,000
    and $55,000 due to parent company)             $  116,399   $  116,226
  Accounts payable                                     90,290       97,516
  Accrued payroll and employee benefits                45,565       59,745
  Accrued income taxes                                 61,855       61,409
  Accrued installation and warranty expenses           38,877       42,404
  Accrued acquisition expenses (Note 6)                23,808       33,789
  Deferred revenue                                     45,296       41,759
  Other accrued expenses                               99,595      101,827
  Due to parent company and affiliated companies        9,638       22,027
                                                   ----------   ----------

                                                      531,323      576,702
                                                   ----------   ----------

Deferred Income Taxes                                  30,438       30,430
                                                   ----------   ----------

Other Deferred Items                                   28,032       27,273
                                                   ----------   ----------

Long-term Obligations:
  Senior convertible obligations (includes
    $140,000 due to parent company)                   327,292      327,824
  Subordinated convertible obligations (Note 2)       404,817      160,547
  Other (includes $53,800 and $168,800 due to
    parent company)                                    68,655      184,823
                                                   ----------   ----------

                                                      800,764      673,194
                                                   ----------   ----------

Minority Interest                                     274,938      165,996
                                                   ----------   ----------

Shareholders' Investment:
  Common stock, $.10 par value, 250,000,000
    shares authorized; 122,822,998 and
    122,645,040 shares issued                          12,282       12,265
  Capital in excess of par value                      332,886      333,580
  Retained earnings                                   647,350      571,899
  Treasury stock at cost, 631,717 and 670,827
    shares                                             (7,247)      (6,965)
  Accumulated other comprehensive items (Note 4)      (33,229)     (33,221)
                                                   ----------   ----------

                                                      952,042      877,558
                                                   ----------   ----------

                                                   $2,617,537   $2,351,153
                                                   ==========   ==========

The accompanying notes are an integral part of these consolidated financial
statements.

                                       3
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.


                        Consolidated Statement of Income
                                   (Unaudited)


                                                       Three Months Ended
                                                       July 4,    June 28,
(In thousands except per share amounts)                   1998        1997
--------------------------------------------------------------------------

Revenues                                              $395,392    $405,235
                                                      --------    --------

Costs and Operating Expenses:
  Cost of revenues                                     204,675     210,494
  Selling, general, and administrative expenses        105,944     111,328
  Research and development expenses                     27,741      27,790
  Restructuring and nonrecurring costs (Note 7)          1,423         800
                                                      --------    --------

                                                       339,783     350,412
                                                      --------    --------

Operating Income                                        55,609      54,823

Interest Income                                          9,381       4,453
Interest Expense (includes $3,023 and $5,059
  to parent company)                                   (12,134)    (11,935)
Gain on Issuance of Stock by Subsidiaries (Note 5)      11,063      13,177
                                                      --------    --------

Income Before Provision for Income Taxes and
  Minority Interest Expense                             63,919      60,518
Provision for Income Taxes                              20,989      20,991
Minority Interest Expense                                5,334       2,308
                                                      --------    --------

Net Income                                            $ 37,596    $ 37,219
                                                      ========    ========

Earnings per Share (Note 3):
  Basic                                               $    .31    $    .31
                                                      ========    ========
  Diluted                                             $    .28    $    .28
                                                      ========    ========

Weighted Average Shares (Note 3):
  Basic                                                122,176     121,527
                                                      ========    ========
  Diluted                                              146,644     139,288
                                                      ========    ========


The accompanying notes are an integral part of these consolidated financial
statements.

                                       4
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.


                        Consolidated Statement of Income
                                   (Unaudited)


                                                         Six Months Ended
                                                         July 4,  June 28,
(In thousands except per share amounts)                     1998      1997
--------------------------------------------------------------------------

Revenues                                                $803,335  $734,355
                                                        --------  --------

Costs and Operating Expenses:
  Cost of revenues                                       418,884   383,942
  Selling, general, and administrative expenses          213,671   200,897
  Research and development expenses                       56,260    51,197
  Restructuring and nonrecurring costs (Note 7)            1,423       800
                                                        --------  --------

                                                         690,238   636,836
                                                        --------  --------

Operating Income                                         113,097    97,519

Interest Income                                           17,550    11,677
Interest Expense (includes $6,454 and $6,618
  to parent company)                                     (23,627)  (20,395)
Gain on Issuance of Stock by Subsidiaries (Note 5)        21,013    25,212
                                                        --------  --------

Income Before Provision for Income Taxes and
  Minority Interest Expense                              128,033   114,013
Provision for Income Taxes                                42,948    38,761
Minority Interest Expense                                  9,634     4,446
                                                        --------  --------

Net Income                                              $ 75,451  $ 70,806
                                                        ========  ========

Earnings per Share (Note 3):
  Basic                                                 $    .62  $    .58
                                                        ========  ========
  Diluted                                               $    .55  $    .53
                                                        ========  ========

Weighted Average Shares (Note 3):
  Basic                                                  122,121   121,433
                                                        ========  ========
  Diluted                                                146,676   139,295
                                                        ========  ========


The accompanying notes are an integral part of these consolidated financial
statements.

                                       5
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.


                      Consolidated Statement of Cash Flows
                                   (Unaudited)


                                                      Six Months Ended
                                                      July 4,   June 28,
(In thousands)                                           1998       1997
------------------------------------------------------------------------

Operating Activities:
  Net income                                        $  75,451  $  70,806
  Adjustments to reconcile net income to net
    cash provided by operating activities:
      Depreciation and amortization                    31,261     26,092
      Provision for losses on accounts receivable         661      2,890
      Decrease in deferred income taxes                     -       (597)
      Gain on issuance of stock by subsidiaries
        (Note 5)                                      (21,013)   (25,212)
      Minority interest expense                         9,634      4,446
      Other noncash expenses                            3,530      2,874
      Changes in current accounts, excluding
        the effects of acquisitions:
          Accounts receivable                          25,565     (9,608)
          Inventories                                 (17,574)   (15,370)
          Other current assets                         (5,471)    (5,983)
          Accounts payable                               (835)    11,425
          Other current liabilities                   (21,349)   (24,599)
      Other                                            (1,376)       (18)
                                                    ---------  ---------

Net cash provided by operating activities              78,484     37,146
                                                    ---------  ---------

Investing Activities:
  Acquisitions, net of cash acquired (Note 6)         (33,084)  (476,420)
  Payment to affiliated company for acquired
    business                                          (19,117)         -
  Proceeds from sale and maturities of
    available-for-sale investments                          -      5,600
  Purchases of property, plant, and equipment         (12,361)   (10,026)
  Proceeds from sale of property, plant, and
    equipment                                           6,621      4,974
  Other, net                                            1,551        579
                                                    ---------  ---------

Net cash used in investing activities               $ (56,390) $(475,293)
                                                    ---------  ---------



                                       6
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.

             Consolidated Statement of Cash Flows (continued)
                                   (Unaudited)


                                                      Six Months Ended
                                                      July 4,   June 28,
(In thousands)                                           1998       1997
------------------------------------------------------------------------

Financing Activities:
  Net proceeds from issuance of Company and
    subsidiary common stock (Note 5)                $ 112,558  $  58,320
  Net proceeds from issuance of subordinated
    convertible debentures (Note 2)                   244,155          -
  Proceeds from issuance of short-term
    obligations to parent company                           -    115,000
  Proceeds from issuance of long-term
    obligations to parent company                           -    220,000
  Repayment of long-term obligation to
    parent company (Note 2)                          (105,000)         -
  Decrease in short-term obligations                  (10,383)    (7,550)
  Repayment of long-term obligations                   (1,657)    (3,930)
  Other                                                    91          -
                                                    ---------  ---------

Net cash provided by financing activities             239,764    381,840
                                                    ---------  ---------

Exchange Rate Effect on Cash                             (567)    (7,053)
                                                    ---------  ---------

Increase (Decrease) in Cash and Cash Equivalents      261,291    (63,360)
Cash and Cash Equivalents at Beginning of Period      468,848    522,688
                                                    ---------  ---------

Cash and Cash Equivalents at End of Period          $ 730,139  $ 459,328
                                                    =========  =========

Noncash Activities:
  Fair value of assets of acquired companies        $  35,982  $ 599,409
  Cash paid for acquired companies                    (33,695)  (518,662)
  Issuance of subsidiary stock options for
    acquired company                                        -     (2,080)
                                                    ---------  ---------

      Liabilities assumed of acquired companies     $   2,287  $  78,667
                                                    =========  =========

  Conversions of Company and subsidiary
    convertible obligations                         $   6,262  $   3,997
                                                    =========  =========


The accompanying notes are an integral part of these consolidated financial
statements.

                                       7
<PAGE>

                         THERMO INSTRUMENT SYSTEMS INC.

                 Notes to Consolidated Financial Statements

1.  General

    The interim consolidated financial statements presented have been prepared
by Thermo Instrument Systems Inc. (the Company) without audit and, in the
opinion of management, reflect all adjustments of a normal recurring nature
necessary for a fair statement of the financial position at July 4, 1998, the
results of operations for the three- and six-month periods ended July 4, 1998,
and June 28, 1997, and the cash flows for the six-month periods ended July 4,
1998, and June 28, 1997. Interim results are not necessarily indicative of
results for a full year.

    The consolidated balance sheet presented as of January 3, 1998, has been
derived from the consolidated financial statements that have been audited by the
Company's independent public accountants. The consolidated financial statements
and notes are presented as permitted by Form 10-Q and do not contain certain
information included in the annual financial statements and notes of the
Company. The consolidated financial statements and notes included herein should
be read in conjunction with the financial statements and notes included in the
Company's Annual Report on Form 10-K for the fiscal year ended January 3, 1998,
filed with the Securities and Exchange Commission.

2.  Subordinated Convertible Debentures

    In January 1998, the Company sold at par value $250.0 million principal
amount of 4% subordinated convertible debentures due 2005 for net proceeds of
$244.2 million. The debentures are convertible into shares of the Company's
common stock at a conversion price of $35.65 per share and are guaranteed on a
subordinated basis by Thermo Electron Corporation. The Company used a portion of
the proceeds to repay a $105.0 million promissory note to Thermo Electron. The
$105.0 million promissory note was classified as long-term in the accompanying
1997 balance sheet, as its repayment was made using the proceeds of debt with a
maturity beyond one year.

3.  Earnings per Share

    Basic and diluted earnings per share were calculated as follows:

                               Three Months Ended      Six Months Ended
                               --------------------   --------------------
(In thousands except           July 4,     June 28,   July 4,     June 28,
per share amounts)                1998         1997      1998         1997
--------------------------------------------------------------------------
Basic
Net income                    $ 37,596     $ 37,219  $ 75,451     $ 70,806
                              --------     --------  --------     --------

Weighted average shares        122,176      121,527   122,121      121,433
                              --------     --------  --------     --------

Basic earnings per share      $    .31     $    .31  $    .62     $    .58
                              ========     ========  ========     ========

                                       8
<PAGE>

3.  Earnings per Share (continued)

                               Three Months Ended      Six Months Ended
                               --------------------   --------------------
(In thousands except           July 4,     June 28,   July 4,     June 28,
per share amounts)                1998         1997      1998         1997
--------------------------------------------------------------------------
Diluted
Net income                    $ 37,596     $ 37,219  $ 75,451     $ 70,806
Effect of:
  Convertible obligations        3,476        2,021     6,953        4,051
  Majority-owned
    subsidiaries' dilutive
    securities                    (736)        (318)   (1,643)        (753)
                              --------     --------  --------     --------

Income available to common
  shareholders, as adjusted   $ 40,336     $ 38,922  $ 80,761     $ 74,104
                              --------     --------  --------     --------

Weighted average shares        122,176      121,527   122,121      121,433
Effect of:
  Convertible obligations       23,446       16,692    23,451       16,760
  Stock options                  1,022        1,069     1,104        1,102
                              --------     --------  --------     --------

Weighted average shares,
  as adjusted                  146,644      139,288   146,676      139,295
                              --------     --------  --------     --------

Diluted earnings per share    $    .28     $    .28  $    .55     $    .53
                              ========     ========  ========     ========

    The computation of diluted earnings per share for each period excludes the
effect of assuming the exercise of certain outstanding stock options because the
effect would be antidilutive. As of July 4, 1998, there were 596,000 of such
options outstanding, with exercise prices ranging from $29.26 to $31.35 per
share.

4.  Comprehensive Income

    During the first quarter of 1998, the Company adopted Statement of Financial
Accounting Standards No. 130, "Reporting Comprehensive Income." This
pronouncement sets forth requirements for disclosure of the Company's
comprehensive income and accumulated other comprehensive items. In general,
comprehensive income combines net income and "other comprehensive items," which
represents certain amounts that are reported as components of shareholders'
investment in the accompanying balance sheet, including foreign currency
translation adjustments and unrealized net-of-tax gains and losses from
available-for-sale investments. During the second quarter of 1998 and 1997, the
Company's comprehensive income totaled $40.7 million and $27.7 million,
respectively. During the first six months of 1998 and 1997, the Company's
comprehensive income totaled $75.4 million and $51.4 million, respectively.


                                       9
<PAGE>

5.  Issuance of Stock by Subsidiaries

    Gain on issuance of stock by subsidiaries in the accompanying statement of
income for the six-month period ended July 4, 1998, resulted from the following:

    Sale of 3,000,000 shares of Thermo BioAnalysis Corporation common stock in a
public offering at $18.125 per share for net proceeds of $51.5 million resulted
in a gain of $8.3 million. In addition, Thermo BioAnalysis sold 1,000,000 shares
of its common stock to Thermo Electron for proceeds, net of commissions, of
$17.5 million, for which no gain was recognized. Following the offering, the
Company owned 60% of Thermo BioAnalysis' outstanding common stock.

    Sale of 3,300,000 shares of ONIX Systems Inc. common stock in an initial
public offering at $14.50 per share for net proceeds of $43.2 million resulted
in a gain of $10.0 million. Following the initial public offering, the Company
owned 68% of ONIX Systems' outstanding common stock.

    Conversion of $1.8 million of Thermo Optek Corporation 5% subordinated
convertible debentures, convertible at $13.94 per share, into 127,646 shares of
Thermo Optek common stock resulted in a gain of $0.9 million. Following the
conversions, the Company owned 91% of Thermo Optek's outstanding common stock.

    Conversion of $4.0 million of ThermoQuest Corporation 5% subordinated
convertible debentures, convertible at $16.50 per share, into 239,393 shares of
ThermoQuest common stock resulted in a gain of $1.8 million. Following the
conversions, the Company owned 87% of ThermoQuest's outstanding common stock.

6.  Acquisitions

    In June 1998, the Company acquired two businesses that constitute a portion
of the environmental group of Smiths Industries plc's Graseby Limited subsidiary
for $22.9 million in cash, net of cash acquired. These businesses are based in
the United Kingdom and provide particulate-monitoring equipment and systems for
the extraction and analysis of samples from industrial smokestacks. During the
first six months of 1998, the Company acquired several other businesses for
$10.2 million in cash, net of cash acquired.

    These acquisitions have been accounted for using the purchase method of
accounting, and their results have been included in the accompanying financial
statements from their respective dates of acquisition. The aggregate cost of
these acquisitions exceeded the estimated fair value of the acquired net assets
by $18.9 million, which is being amortized over 40 years. Allocation of the
purchase price for these acquisitions was based on estimates of the fair value
of the net assets acquired and is subject to adjustment upon finalization of the
purchase price allocation. The Company has gathered no information that
indicates the final allocations will differ materially from the preliminary
estimates. Pro forma results have not been presented as the results of the
acquired businesses were not material to the Company's results of operations.

                                       10
<PAGE>


6.  Acquisitions (continued)

    During 1997, the Company undertook a restructuring of Life Sciences
International PLC, acquired in March 1997. In March 1998, the Company finalized
its plan for restructuring the acquired business. At January 3, 1998, the
remaining reserve for restructuring activities totaled $10.2 million. During the
first six months of 1998, the Company expended $4.5 million for restructuring
costs, primarily for severance. At July 4, 1998, the remaining reserve for
restructuring the Life Sciences businesses was $7.4 million, as adjusted for the
impact of currency translation, and primarily represents ongoing severance and
abandoned-facility payments.

    During 1996, the Company undertook a restructuring of a substantial portion
of the businesses constituting the Scientific Instruments Division of Fisons
plc, acquired in March 1996. In March 1997, the Company finalized its plan for
restructuring the acquired businesses. At January 3, 1998, the remaining reserve
for restructuring activities totaled $11.1 million. During the first six months
of 1998, the Company expended $1.2 million for restructuring costs, primarily
for severance and abandoned-facility payments. At July 4, 1998, the remaining
reserve for restructuring the Fisons businesses was $9.9 million and primarily
represents ongoing severance and abandoned-facility payments.

7.   Restructuring and Nonrecurring Costs

    In December 1996, five former employees of the Company's Epsilon Industrial,
Inc. subsidiary sought damages in an arbitration proceeding for alleged breaches
of agreements entered into with such employees prior to Epsilon's acquisition by
the Company. The arbitrators rendered a decision with respect to such claims
during the second quarter of 1998 and the Company recorded $1.4 million of
nonrecurring costs related to the resolution of this matter.

Item 2 - Management's Discussion and Analysis of Financial Condition and
         Results of Operations

    Forward-looking statements, within the meaning of Section 21E of the
Securities Exchange Act of 1934, are made throughout this Management's
Discussion and Analysis of Financial Condition and Results of Operations. For
this purpose, any statements contained herein that are not statements of
historical fact may be deemed to be forward-looking statements. Without limiting
the foregoing, the words "believes," "anticipates," "plans," "expects," "seeks,"
"estimates," and similar expressions are intended to identify forward-looking
statements. There are a number of important factors that could cause the results
of the Company to differ materially from those indicated by such forward-looking
statements, including those detailed under the heading "Forward-looking
Statements" in Exhibit 13 to the Company's Annual Report on Form 10-K for the
fiscal year ended January 3, 1998, filed with the Securities and Exchange
Commission.


                                       11
<PAGE>

Results of Operations

Second Quarter 1998 Compared With Second Quarter 1997

     Revenues decreased $9.8 million, or 2%, to $395.4 million in the second
quarter of 1998 from $405.2 million in the second quarter of 1997. Revenues
decreased primarily due to an $18.4 million decline in sales at ThermoQuest
Corporation. ThermoQuest's sales decline resulted principally from a $15.6
million decrease in sales of analytical instruments, of which $9.0 million was
attributable to a decline in revenues from Europe and North America, primarily
due to orders being received late in the quarter, and $4.0 million was
attributable to lower sales in Asia due to unstable economic conditions in that
region. The remainder of the decrease in ThermoQuest's sales resulted primarily
from the impact of an affiliated company no longer using ThermoQuest's sales
offices as a distributor for certain products and the unfavorable effects of
currency translation due to the strengthening of the U.S. dollar relative to
foreign currencies in countries in which it operates. ThermoQuest's backlog at
July 4, 1998, increased by $7.2 million from the end of the first quarter of
1998. In addition, the effect of lower sales at Thermo Optek Corporation,
primarily due to lower sales to the semiconductor industry and lower sales in
Asia, was offset in part by higher sales at ONIX Systems Inc. due to the
inclusion of revenues from acquisitions and increased sales of industry-specific
instruments to the production segment of the oil and gas industry. In total,
acquisitions added $15.0 million of revenues in 1998 and the unfavorable effects
of currency translation decreased revenues by $7.2 million in 1998.

    The Company's backlog decreased $29.1 million during the first six months of
1998 to $269.8 million. The backlog decreased primarily at Thermo Optek and at
ThermoSpectra Corporation and was principally a result of a slowdown in the
semiconductor and related industries.

    International sales account for a significant portion of the Company's total
revenues. Although the Company seeks to charge its customers in the same
currency as its operating costs, the Company's financial performance and
competitive position can be affected by currency exchange rate fluctuations.
Where appropriate, the Company uses forward exchange contracts to reduce its
exposure to currency fluctuations.

    The gross profit margin was unchanged at 48% in the second quarter of 1998
and 1997.

    Selling, general, and administrative expenses as a percentage of revenues
was unchanged at 27% in the second quarter of 1998 and 1997. Research and
development expenses as a percentage of revenues was unchanged at 7% in 1998 and
1997.

    Five former employees of the Company's Epsilon Industrial, Inc.
subsidiary had sought damages in an arbitration proceeding for alleged
breaches of agreements entered into with such employees prior to Epsilon's
acquisition by the Company. The arbitrators rendered a decision

                                       12
<PAGE>

Second Quarter 1998 Compared With Second Quarter 1997 (continued)

with respect to such claims during the second quarter of 1998 and the Company
recorded $1.4 million of nonrecurring costs related to the resolution of this
matter (Note 7). Restructuring and nonrecurring costs of $0.8 million in 1997
represents severance for employees terminated during the quarter at one of
ThermoSpectra's business units. ThermoSpectra expects to undertake additional
restructuring activities during the remainder of 1998 which will result in
additional charges.

    Interest income increased to $9.4 million in the second quarter of 1998 from
$4.5 million in the second quarter of 1997, primarily due to interest income
earned on invested proceeds from the Company's January 1998 issuance of $250.0
million principal amount of 4% subordinated convertible debentures, offset in
part by the use of a portion of the proceeds to repay a $105.0 million
promissory note to Thermo Electron Corporation (Note 2). To a lesser extent,
interest income increased due to higher invested cash balances as a result of
the sale of common stock by the Company's subsidiaries in 1997 and 1998.
Interest expense increased slightly to $12.1 million in 1998 from $11.9 million
in 1997.

    The Company has adopted a strategy of spinning out certain of its businesses
into separate subsidiaries and having these subsidiaries sell a minority
interest to outside investors. The Company believes that this strategy provides
additional motivation and incentives for the management of the subsidiaries
through the establishment of subsidiary-level stock option programs, as well as
capital to support the subsidiaries' growth. As a result of the sale of stock by
subsidiaries and issuance of stock by subsidiaries upon conversion of
convertible debentures, the Company recorded gains of $11.1 million in the
second quarter of 1998 (Note 5) and $13.2 million in the second quarter of 1997.
The size and timing of these transactions are dependent on market and other
conditions that are beyond the Company's control. Accordingly, there can be no
assurance that the Company will be able to realize gains from such transactions
in the future.

    The effective tax rate was 33% in the second quarter of 1998 and 35% in the
second quarter of 1997. Excluding the impact of the nontaxable gains on issuance
of stock by subsidiaries in 1998 and 1997, the effective tax rates in both
periods exceeded the statutory federal income tax rate due to nondeductible
amortization of cost in excess of net assets of acquired companies, the
inability to provide a tax benefit on losses incurred at certain foreign
subsidiaries, and the impact of state income taxes. Excluding the impact of the
nontaxable gains, the effective tax rate decreased in 1998, primarily due to the
lower relative effect of nondeductible amortization of cost in excess of net
assets of acquired companies and, to a lesser extent, a decrease in foreign tax
losses not benefited.

    Minority interest expense increased to $5.3 million in the second quarter of
1998 from $2.3 million in the second quarter of 1997, primarily due to the
minority interest associated with the Company's

                                       13
<PAGE>

Second Quarter 1998 Compared With Second Quarter 1997 (continued)

newly public ONIX Systems and Thermo Vision Corporation subsidiaries, the
minority interest associated with the earnings of certain of the Life Sciences
International PLC businesses sold to ThermoQuest and Thermo Optek in 1997, and
the increase in minority ownership as a result of the June 1998 sale of common
stock by Thermo BioAnalysis.

    The Company is currently assessing the potential impact of the year 2000 on
the processing of date-sensitive information by the Company's computerized
information systems and on products sold as well as products purchased by the
Company. The Company believes that its internal information systems and current
products are either year 2000 compliant or will be so prior to the year 2000
without incurring material costs. There can be no assurance, however, that the
Company will not experience unexpected costs and delays in achieving year 2000
compliance for its internal information systems and current products, which
could result in a material adverse effect on the Company's future results of
operations.

    The Company is presently assessing the effect that the year 2000 issue may
have on its previously sold products. The Company is also assessing whether its
key suppliers are adequately addressing this issue and the effect this might
have on the Company. The Company has not completed its analysis and is unable to
conclude at this time that the year 2000 issue as it relates to its previously
sold products and products purchased from key suppliers is not reasonably likely
to have a material adverse effect on the Company's future results of operations.

First Six Months 1998 Compared With First Six Months 1997

    Revenues increased $69.0 million, or 9%, to $803.3 million in the first six
months of 1998 from $734.4 million in the first six months of 1997, due to
acquisitions, which included Life Sciences in March 1997. Acquisitions added
revenues of $96.6 million in the first six months of 1998. Sales increased at
ONIX Systems due to higher sales of industry-specific instruments to the
production segment of the oil and gas industry and at Metrika Systems
Corporation as a result of higher demand at its finished-materials business and,
to a lesser extent, at its raw materials business. The increase in revenues was
offset in part by a decrease of $15.7 million in revenues due to the unfavorable
effects of currency translation as a result of the strengthening of the U.S.
dollar relative to foreign currencies in countries in which the Company
operates. At ThermoQuest, revenues from Asia decreased $12.0 million due to
economic uncertainty in that region. Revenues decreased at Thermo Optek due to
lower sales to the semiconductor industry and lower sales in Asia.

    The gross profit margin was unchanged at 48% in the first six months of 1998
and 1997. A decline in the gross profit margin in 1998 due to the inclusion of
lower-margin revenues at certain businesses acquired in 1997 was offset by the
impact in 1997 of an adjustment to expense of $3.2 million relating to the sale
of inventories revalued at the time of the acquisition of Life Sciences.



                                       14
<PAGE>

First Six Months 1998 Compared With First Six Months 1997 (continued)

    Selling, general, and administrative expenses as a percentage of revenues
was unchanged at 27% in the first six months of 1998 and 1997. Research and
development expenses as a percentage of revenues was unchanged at 7% in 1998 and
1997.

    Interest income increased to $17.6 million in the first six months of 1998
from $11.7 million in the first six months of 1997, primarily due to the reasons
discussed in the results of operations for the second quarter. Interest expense
increased to $23.6 million in 1998 from $20.4 million in 1997, primarily due to
the issuance of an aggregate $428.8 million of promissory notes to Thermo
Electron in 1997 in connection with acquisitions and the January 1998 issuance
of the 4% subordinated convertible debentures.

    As a result of the sale of stock by subsidiaries and issuance of stock by
subsidiaries upon conversion of convertible debentures, the Company recorded
gains of $21.0 million in the first six months of 1998 (Note 5)
and $25.2 million in the first six months of 1997.

    The effective tax rate was 34% in the first six months of 1998 and 1997.
Excluding the impact of the nontaxable gains on issuance of stock by
subsidiaries in 1998 and 1997, the effective tax rates in both periods exceeded
the statutory federal income tax rate due to nondeductible amortization of cost
in excess of net assets of acquired companies, the inability to provide a tax
benefit on losses incurred at certain foreign subsidiaries, and the impact of
state income taxes. Excluding the impact of the nontaxable gains, the effective
tax rate decreased in 1998, primarily due to the lower relative effect of
nondeductible amortization of cost in excess of net assets of acquired companies
and, to a lesser extent, a decrease in foreign tax losses not benefited.

    Minority interest expense increased to $9.6 million in the first six months
of 1998 from $4.4 million in the first six months of 1997, primarily due to the
minority interest associated with the Company's newly public ONIX Systems,
Metrika Systems, and Thermo Vision subsidiaries and the earnings of certain of
the Life Sciences businesses sold to ThermoQuest and Thermo Optek in 1997.


                                       15
<PAGE>

Liquidity and Capital Resources

    Consolidated working capital was $922.7 million at July 4, 1998, compared
with $612.7 million at January 3, 1998. Included in working capital are cash,
cash equivalents, and available-for-sale investments of $739.2 million at July
4, 1998, and $477.2 million at January 3, 1998. Of the $739.2 million balance at
July 4, 1998, $471.9 million was held by the Company's majority-owned
subsidiaries and the balance was held by the Company and its wholly owned
subsidiaries. The Company's operating activities provided cash of $78.5 million
in the first six months of 1998. The Company used cash of $21.3 million to
reduce other current liabilities, primarily to reduce accrued payroll and
related benefits, principally at ThermoQuest, and to make payments for accrued
acquisition expenses (Note 6). The Company used cash of $17.6 million to
increase inventories, in part at Thermo BioAnalysis as a result of an increase
in finished-goods inventory at its liquid-handling business and in part at
ThermoQuest to replenish year-end inventory levels at its European sales offices
and to build up inventories in preparation for a new product release. Cash flow
from operations was improved by a decrease in accounts receivable of $25.6
million, primarily due to lower revenues at ThermoQuest and Thermo Optek,
management efforts at Thermo Optek to reduce its investment in accounts
receivable, and the timing of cash collections at Metrika Systems.

    At July 4, 1998, $135.0 million of the Company's cash and cash equivalents
was held by its foreign subsidiaries. While this cash can be used outside of the
United States, including for acquisitions, repatriation of this cash into the
United States would be subject to foreign withholding taxes and could also be
subject to a United States tax.

    The Company's investing activities used $56.4 million of cash in the first
six months of 1998, including $33.1 million for acquisitions (Note 6). ONIX
Systems used $19.1 million of cash to pay Thermo Power Corporation for the Peek
Measurement Business, acquired effective November 1997. The Company expended
$12.4 million for purchases of property, plant, and equipment and received
proceeds of $6.6 million from the sale of property, plant, and equipment.

    The Company's financing activities provided $239.8 million of cash in the
first six months of 1998. Net proceeds from the issuance of Company and
subsidiary common stock totaled $112.6 million, which included $112.2 million of
net proceeds from the June 1998 sale of Thermo BioAnalysis' common stock and the
March 1998 initial public offering of ONIX Systems' common stock (Note 5). In
January 1998, the Company sold at par value $250.0 million principal amount of
4% subordinated convertible debentures due 2005 for net proceeds of $244.2
million (Note 2). The Company used a portion of the proceeds to repay a $105.0
million promissory note to Thermo Electron.

    In July 1998, the Company's Board of Directors authorized the repurchase by
the Company of up to 3 million shares of its common stock, through July 1999, in
the open market or in negotiated transactions, as well as up to $25 million of
the common stock of its subsidiaries.


                                       16
<PAGE>

Liquidity and Capital Resources (continued)

    During the remainder of 1998, the Company plans to make expenditures of
approximately $18 million for property, plant, and equipment. Since July 4,
1998, the Company and its majority-owned subsidiaries have expended $44.5
million on acquisitions of businesses and as of August 12, 1998, the Company and
its majority-owned subsidiaries had agreements or nonbinding letters of intent
to acquire new businesses totaling approximately $32 million. Proposed
acquisitions of new businesses are subject to various conditions to closing, and
there can be no assurance that all proposed transactions will be consummated.
The Company believes that its existing resources are sufficient to meet the
capital requirements of its existing operations for the foreseeable future. The
Company has historically complemented internal development with acquisitions of
businesses or technologies that extend the Company's presence in current markets
or provide opportunities to enter and compete effectively in new markets. The
Company will consider making acquisitions of such businesses or technologies
that are consistent with its plans for strategic growth. The Company expects
that it will finance these acquisitions through a combination of internal funds
and/or short-term borrowings from Thermo Electron, although there is no
agreement with Thermo Electron to ensure that funds will be available on
acceptable terms or at all.

Market Risk

    The Company's exposure to market risk from changes in foreign currency
exchange rates, interest rates, and equity prices has not changed materially
from its exposure at year-end 1997.

PART II - OTHER INFORMATION

Item 4 - Submission of Matters to a Vote of Security Holders

    On June 1, 1998, at the Annual Meeting of Shareholders, the
shareholders elected six directors to a one-year term expiring in 1999.
The directors elected at the meeting were: Col. Frank Borman, Dr. George
N. Hatsopoulos, Mr. John N. Hatsopoulos, Mr. Earl R. Lewis, Mr. Arvin H.
Smith, and Mr. Polyvios C. Vintiadis. Col. Borman received 117,173,601
shares voted in favor of election and 81,202 shares voted against; Dr. G.
Hatsopoulos and Mr. J. Hatsopoulos each received 117,181,941 shares voted
in favor of election and 72,862 shares voted against; Mr. Lewis received
117,181,993 shares voted in favor of election and 72,810 shares voted
against; Mr. Smith received 117,182,066 shares voted in favor of election
and 72,737 shares voted against; and Mr. Vintiadis received 117,181,696
shares voted in favor of election and 73,107 shares voted against. No
abstentions or broker nonvotes were recorded on the election of directors.


                                       17
<PAGE>

Item 5 - Other Information

    Pursuant to recent amendments to the rules relating to proxy statements
under the Securities Exchange Act of 1934, as amended (the Exchange Act),
shareholders of the Company are hereby notified that any shareholder proposal
not included in the Company's proxy materials for its 1999 Annual Meeting of
Shareholders (the Annual Meeting) in accordance with Rule 14a-8 under the
Exchange Act will be considered untimely for the purposes of Rules 14a-4 and
14a-5 under the Exchange Act if notice thereof is received by the Company after
March 15, 1999. Management proxies will be authorized to exercise discretionary
voting authority with respect to any shareholder proposal not included in the
Company's proxy materials for the Annual Meeting unless (a) the Company receives
notice of such proposal by March 15, 1999, and (b) the conditions set forth in
Rule 14a-4(c)(2)(i)-(iii) under the Exchange Act are met.

    On August 12, 1998, Thermo Electron Corporation, which owns a majority of
the Company's outstanding common stock, issued a press release concerning a
proposed corporate reorganization involving several of its subsidiaries,
including the Company. See Exhibit 99.

Item 6 - Exhibits

    See Exhibit Index on the page immediately preceding exhibits.



                                       18
<PAGE>

                                   SIGNATURES


    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 as of the 12th day of August 1998.

                                       THERMO INSTRUMENT SYSTEMS INC.



                                       Paul F. Kelleher
                                       ---------------------------
                                       Paul F. Kelleher
                                       Chief Accounting Officer



                                       John N. Hatsopoulos
                                       ---------------------------
                                       John N. Hatsopoulos
                                       Chief Financial Officer and
                                         Senior Vice President

                                       19
<PAGE>

                                     
                                  EXHIBIT INDEX


Exhibit
Number        Description of Exhibit
------------------------------------------------------------------------
  27          Financial Data Schedule.

  99          Thermo Electron Corporation Press Release dated August 12,
              1998.

                                       20
<PAGE>