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)


                 AMENDED AND RESTATED 1995 STOCK INCENTIVE 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 (1) Share Price Registration Fee - ----------------- ------------- ----- ----- ---------------- Common Stock, no 5,250,000 $22.86 $120,015,000 $11,041.38 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 Statements on Form S-8, File No. 333-78071 and 333-54490, relating to the Registrant's Amended and Restated 1995 Stock Incentive Plan, as Amended. 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 - ---------------------- Executive Officer (Principal John R. Bertucci Executive Officer) /s/ Ronald C. Weigner Vice President and Chief June 14, 2002 - ---------------------- Financial Officer (Principal Ronald C. Weigner Financial Officer and Principal Accounting Officer) /s/ Robert R. Anderson Director June 14, 2002 - ---------------------- Robert R. Anderson /s/ James G. Berger 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).
- -------------------------------------------------------------------------------- (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.

                                                                     Exhibit 5.1

                               HALE AND DORR LLP
                               COUNSELLORS AT LAW


                                  HALEDORR.COM
                       60 STATE STREET - BOSTON, MA 02109
                        617-526-6000 - FAX 617-526-5000



June 14, 2002


MKS Instruments, Inc.
Six Shattuck Road
Andover, MA 01810


         Re:      Amended and Restated 1995 Stock Incentive Plan, as amended

Ladies and Gentlemen:

         We have assisted in the preparation of a Registration Statement on Form
S-8 (the "Registration Statement") to be filed with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to an aggregate of 5,250,000 shares of common stock,
no par value per share (the "Shares"), of MKS Instruments, Inc., a Massachusetts
corporation (the "Company"), issuable under the Company's Amended and Restated
1995 Stock Incentive Plan, as amended (the "Plan").

         We have examined the Articles of Organization and By-Laws of the
Company, each as amended and restated to date, and originals, or copies
certified to our satisfaction, of all pertinent records of the meetings of the
directors and stockholders of the Company, the Registration Statement and such
other documents relating to the Company as we have deemed material for the
purposes of this opinion.

         In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as certified, photostatic or other copies, the authenticity of the originals
of any such documents and the legal competence of all signatories to such
documents.

         We assume that the appropriate action will be taken, prior to the offer
and sale of the shares in accordance with the Plan, to register and qualify the
shares for sale under all applicable state securities or "blue sky" laws.

MKS Instruments, Inc.
June 14, 2002
Page 2


         We express no opinion herein as to the laws of any state or
jurisdiction other than the state laws of The Commonwealth of Massachusetts and
the federal laws of the United States of America.

         It is understood that this opinion is to be used only in connection
with the offer and sale of the Shares while the Registration Statement is in
effect.

         Please note that we are opining only as to the matters expressly set
forth herein, and no opinion should be inferred as to any other matters.

         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized for issuance and, when the Shares are issued and paid for in
accordance with the terms and conditions of the Plan, the Shares will be validly
issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion with the Securities and
Exchange Commission in connection with the Registration Statement in accordance
with the requirements of Item 601(b)(5) of Regulation S-K under the Securities
Act. In giving such consent, we do not hereby admit that we are in the category
of persons whose consent is required under Section 7 of the Securities Act or
the rules and regulations of the Commission.

                                                     Very truly yours,



                                                     /s/ HALE AND DORR LLP

                                                     HALE AND DORR LLP


                                                                    Exhibit 23.2



                    CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS

We hereby consent to the incorporation by reference in this Registration
Statement on Form S-8 of our report dated January 31, 2002 relating to the
financial statements and financial statement schedule of MKS Instruments, Inc.,
which appears in MKS Instruments, Inc.'s Annual Report on Form 10-K for the year
ended December 31, 2001.


/s/ PricewaterhouseCoopers LLP

Boston, Massachusetts
June 13, 2002



                                                                    Exhibit 23.3

                          Independent Auditors' Consent

The Board of Directors
Applied Science and Technology, Inc and Subsidiaries:

We consent to the use of our report dated July 31, 2000, with respect to the
consolidated balance sheet of Applied Science and Technology, Inc., and
subsidiaries as of July 1, 2000, and the related consolidated statements of
operations, stockholders' equity, and cash flows for each of the years in the
two-year period ended July 1, 2000, incorporated herein by reference.


/s/ KPMG LLP

Boston, MA
June 13, 2002