As filed with the Securities and Exchange Commission on June 14, 2002

                                                      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                                          04-2277512
(State or Other Jurisdiction of                              (I.R.S. Employer
Incorporation or Organization)                            Identification Number)

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


             INTERNATIONAL EMPLOYEE STOCK PURCHASE PLAN, AS AMENDED
                            (Full Title of the Plan)

                                JOHN R. BERTUCCI
                      CHAIRMAN AND CHIEF EXECUTIVE OFFICER
                              MKS INSTRUMENTS, INC.
                                SIX SHATTUCK ROAD
                                ANDOVER, MA 01810
                     (Name and Address of Agent For Service)

                                 (978) 975-2350
          (Telephone Number, Including Area Code, of Agent For Service)

                         CALCULATION OF REGISTRATION FEE



                                                  Proposed Maximum       Proposed Maximum
Title of Securities         Amount to be         Offering Price Per     Aggregate Offering          Amount of
to be Registered           Registered(1)                Share                  Price            Registration Fee
-------------------        -------------         ------------------     ------------------      ----------------
                                                                                    
Common Stock, no              25,000                   $22.86                $571,500                $52.58
par value per share


(1) Estimated solely for the purpose of calculating the registration fee
pursuant to Rules 457(c) and 457(h) of the Securities Act of 1933, as amended,
and based upon the average of the high and low prices of the Registrant's Common
Stock as reported on the Nasdaq National Market on June 12, 2002.

                     STATEMENT OF INCORPORATION BY REFERENCE

         Except as otherwise set forth below, this Registration Statement on
Form S-8 incorporates by reference the contents of the Registration Statement on
Form S-8, File No. 333-31224, relating to the Registrant's International
Employee Stock Purchase Plan.

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, 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 Andover, Massachusetts on June 14, 2002.

                            MKS INSTRUMENTS, INC.


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


                        POWER OF ATTORNEY AND SIGNATURES

         We, the undersigned officers and directors of MKS Instruments, Inc.
hereby severally constitute and appoint 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, THIS REGISTRATION
STATEMENT HAS BEEN SIGNED BY THE FOLLOWING PERSONS IN THE CAPACITIES AND ON THE
DATES INDICATED.



        Signature                                         Title                         Date
        ---------                                         -----                         ----
                                                                              
 /s/ John R. Bertucci                           Chairman of the Board and Chief     June 14, 2002
---------------------
John R. Bertucci                                Executive Officer (Principal
                                                Executive Officer)

 /s/ Ronald C. Weigner                          Vice President and Chief            June 14, 2002
----------------------
Ronald C. Weigner                               Financial Officer (Principal
                                                Financial Officer and Principal
                                                Accounting Officer)

 /s/ Robert R. Anderson                         Director                            June 14, 2002
-------------------------
Robert R. Anderson

 /s/ James G. Berges                            Director                            June 14, 2002
-------------------------
James G. Berges

 /s/ Richard S. Chute                           Director                            June 14, 2002
--------------------------
Richard S. Chute



                                                                              
 /s/ Hans-Jochen Kahl                           Director                            June 14, 2002
-------------------------
Hans-Jochen Kahl

 /s/ Owen W. Robbins                            Director                            June 14, 2002
------------------------
Owen W. Robbins

 /s/ Louis P. Valente                           Director                            June 14, 2002
--------------------------
Louis P. Valente


                                  EXHIBIT INDEX

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

4.1 (1)    Restated Articles of Organization of the Registrant.

4.2 (2)    Articles of Amendment.

4.3 (3)    Articles of Amendment.

4.4 (4)    Amended and Restated By-Laws of the Registrant.

4.5 (4)    Specimen Certificate for Common Stock of the Registrant.

5.1        Opinion of Hale and Dorr LLP.

23.1       Consent of Hale and Dorr LLP (included in Exhibit 5.1).

23.2       Consent of PricewaterhouseCoopers LLP.

23.3       Consent of KPMG LLP.

24.1       Power of Attorney (included in the signature pages of this
           Registration Statement).

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(1)       Incorporated herein by reference from the Registrant's Registration
          Statement on Form S-4 (File No. 333-49738) originally filed with the
          Securities and Exchange Commission on December 13, 2000, as amended.

(2)       Incorporated herein by reference from the Registrant's Quarterly
          Report on Form 10-Q originally filed with the Securities and Exchange
          Commission on August 14, 2001.

(3)       Incorporated herein by reference from the Registrant's Definitive
          Proxy originally filed with the Securities and Exchange Commission on
          April 16, 2002.

(4)       Incorporated herein by reference from the Registrant's Registration
          Statement on Form S-1 (File No. 333-71363) originally filed with the
          Securities and Exchange Commission on January 28, 1999, as amended.