As filed with the Securities and Exchange Commission on August 5, 2005

                                                      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)

90 INDUSTRIAL WAY, WILMINGTON, MASSACHUSETTS                     01887
(Address of Principal Executive Offices)                       (Zip Code)

                            2004 STOCK INCENTIVE PLAN
                            (Full Title of the Plan)

                                RONALD C. WEIGNER
                   VICE PRESIDENT AND CHIEF FINANCIAL OFFICER
                              MKS INSTRUMENTS, INC.
                                90 INDUSTRIAL WAY
                              WILMINGTON, MA 01887
                     (Name and Address of Agent For Service)

                                 (978) 284-4000
          (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      15,000,000        $18.83(2)         $282,450,000(2)       $33,245
par value per share


(1)   In accordance with Rule 416 under the Securities Act of 1933, as amended,
      this registration statement shall be deemed to cover any additional
      securities that may from time to time be offered or issued to prevent
      dilution resulting from stock splits, stock dividends or similar
      transactions.

(2)   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
      August 2, 2005.



                                     PART I
              INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

      ITEM 1. PLAN INFORMATION.

      The information required by Item 1 is included in documents sent or given
to participants in the plans covered by this registration statement pursuant to
Rule 428(b)(1) of the Securities Act of 1933, as amended (the "Securities Act").

      ITEM 2. REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION.

      The written statement required by Item 2 is included in documents sent or
given to participants in the plans covered by this registration statement
pursuant to Rule 428(b)(1) of the Securities Act.

                                    PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

      ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

      The registrant is subject to the informational and reporting requirements
of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), and in accordance therewith files reports, proxy
statements and other information with the Securities and Exchange Commission
(the "Commission"). The following documents, which are on file with the
Commission, are incorporated in this registration statement by reference:

      (a) The registrant's latest annual report filed pursuant to Section 13(a)
or 15(d) of the Exchange Act or the latest prospectus filed pursuant to Rule
424(b) under the Securities Act that contains audited financial statements for
the registrant's latest fiscal year for which such statements have been filed.

      (b) All other reports filed pursuant to Section 13(a) or 15(d) of the
Exchange Act since the end of the fiscal year covered by the document referred
to in (a) above.

      (c) The description of the securities contained in the registrant's
registration statements on Form 8-A filed under the Exchange Act, including any
amendment or report filed for the purpose of updating such description.

      All documents subsequently filed by the registrant pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a
post-effective amendment which indicates that all securities offered hereby have
been sold or which deregisters all securities then remaining unsold, shall be
deemed to be incorporated by reference in this registration statement and to be
part hereof from the date of the filing of such documents. Any statement
contained in a document incorporated or deemed to be incorporated by reference
herein shall be deemed to be modified or superseded for the purposes of this
registration statement to the extent that a statement contained herein or in any
other subsequently filed document which also is or is deemed to be incorporated
by reference herein modifies or supersedes such statement. Any statement so
modified or superseded shall not be deemed, except as so modified or superseded,
to constitute a part of this registration statement.

      ITEM 4. DESCRIPTION OF SECURITIES.

      Not applicable.

                                     - 2 -


      ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

      Wilmer Cutler Pickering Hale and Dorr LLP has opined as to the legality of
the securities being offered by this registration statement.

      ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

      Section 8.51(a) of Chapter 156D of the Massachusetts General Laws provides
that a corporation may indemnify its directors against expenses (including
attorneys' fees), judgments, fines and amounts paid in settlement reasonably
incurred in connection with any litigation or other legal proceeding brought
against any director by virtue of his position as a director of the registrant
unless he is deemed to have not acted in good faith in the reasonable belief
that his action was in the best interest of the corporation.

      Section 8.52 of Chapter 156D provides that a corporation must indemnify a
director who is wholly successful, on the merits or otherwise, in the defense of
any proceeding to which the director was a party because he was a director of
the corporation against reasonable expenses incurred by him in connection with
the proceeding.

      Section 8.56(a) of Chapter 156D provides that a corporation may indemnify
its officers to the same extent as its directors and, for officers that are not
directors, to the extent provided by (i) the articles of organization, (ii) the
bylaws, (iii) a vote of the board of directors or (iv) a contract. In all
instances, the extent to which a corporation provides indemnification to its
officers under Section 8.56(a) is optional. Section 8.56(c) provides that an
officer of a corporation who is not a director is entitled to mandatory
indemnification under Section 8.52.

      In its Restated Articles of Organization (the "Articles of Organization"),
the registrant has elected to commit to provide indemnification to its directors
and officers in specified circumstances. Generally, Article 6 of the
registrant's Articles of Organization provides that the registrant shall
indemnify directors and officers of the registrant against liabilities and
expenses arising out of legal proceedings brought against them by reason of
their status as directors or officers or by reason of their agreeing to serve,
at the request of the registrant, as a director or officer with another
organization. Under this provision, a director or officer of the registrant
shall be indemnified by the registrant for all expenses (including attorneys'
fees), judgments and fines, and amounts paid in settlement of such proceedings,
even if he is not successful on the merits, unless finally adjudicated not to
have acted in good faith in the reasonable belief that his action was in the
best interests of the registrant. The registrant shall not indemnify an officer
or director who initiates the legal proceeding, unless the initiation was
approved by the Board of Directors. The Board of Directors may authorize
advancing litigation expenses to a director or officer at his request upon
receipt of an undertaking by any such director or officer to repay such expenses
if it is ultimately determined that he is not entitled to indemnification for
such expenses.

      Section 2.02(b)(4) of Chapter 156D of the Massachusetts General Laws
allows a corporation to eliminate or limit the personal liability of a director
of a corporation to the corporation for monetary damages for a breach of
fiduciary duty as a director notwithstanding any provision of law imposing such
liability, except where the director breached his duty of loyalty to the
corporation or its shareholders, failed to act in good faith, engaged in
intentional misconduct or knowingly violated a law, authorized the payment of an
improper distribution or obtained an improper personal benefit. Article 6A of
the registrant's Articles of Organization eliminates the personal liability of
the registrant's directors to the registrant or its stockholders for monetary
damages for breach of a director's fiduciary duty, except to the extent Chapter
156D of the Massachusetts General Laws prohibits the elimination or limitation
of such liability.

                                     - 3 -


      The registrant has obtained directors and officers' liability insurance
for the benefit of its directors and certain of its officers.

      ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

      Not applicable.

      ITEM 8. EXHIBITS.

      The Exhibit Index immediately preceding the exhibits is incorporated
herein by reference.

      ITEM 9. UNDERTAKINGS.

      1.    Item 512(a) of Regulation S-K. The undersigned registrant hereby
            undertakes:

            (1)   To file, during any period in which offers or sales are being
      made, a post-effective amendment to this registration statement:

                  (i) To include any prospectus required by Section 10(a)(3) of
      the Securities Act;

                  (ii) To reflect in the prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent a fundamental change in the information set forth in the
      registration statement; and

                  (iii) To include any material information with respect to the
      plan of distribution not previously disclosed in the registration
      statement or any material change to such information in the registration
      statement;

      provided, however, that paragraphs (i) and (ii) do not apply if the
      information required to be included in a post-effective amendment by those
      paragraphs is contained in periodic reports filed with or furnished to the
      Commission by the registrant pursuant to Section 13 or Section 15(d) of
      the Exchange Act that are incorporated by reference in the registration
      statement.

            (2)   That, for the purpose of determining any liability under the
      Securities Act, each such post-effective amendment shall be deemed to be a
      new registration statement relating to the securities offered therein, and
      the offering of such securities at that time shall be deemed to be the
      initial bona fide offering thereof.

            (3)   To remove from registration by means of a post-effective
      amendment any of the securities being registered which remain unsold at
      the termination of the offering.

      2.    Item 512(b) of Regulation S-K. The undersigned registrant hereby
undertakes that, for purposes of determining any liability under the Securities
Act, each filing of the registrant's annual report pursuant to Section 13(a) or
Section 15(d) of the Exchange Act that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

                                     - 4 -


      3.    Item 512(h) of Regulation S-K. Insofar as indemnification for
liabilities arising under the Securities Act may be permitted to directors,
officers and controlling persons of the registrant pursuant to the foregoing
provisions, or otherwise, the registrant has been advised that in the opinion of
the Securities and Exchange Commission such indemnification is against public
policy as expressed in the Securities Act and is, therefore, unenforceable. In
the event that a claim for indemnification against such liabilities (other than
the payment by the registrant of expenses incurred or paid by a director,
officer or controlling person of the registrant in the successful defense of any
action, suit or proceeding) is asserted by such director, officer or controlling
person in connection with the securities being registered, the registrant will,
unless in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act and will be governed by the final adjudication of such issue.

                                     - 5 -


                                   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 Wilmington, Massachusetts on August 1, 2005.

                             MKS INSTRUMENTS, INC.

                             By: /s/ Leo Berlinghieri
                                 ------------------------
                                 Leo Berlinghieri
                                 President, Chief Executive Officer and Director

                        POWER OF ATTORNEY AND SIGNATURES

      We, the undersigned officers and directors of MKS Instruments, Inc. hereby
severally constitute and appoint Leo Berlinghieri, Ronald C. Weigner and
Kathleen F. Burke, 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
     ---------                   -----                        ----
                                                   
                        President, Chief Executive       August 1, 2005
/s/ Leo Berlinghieri    Officer and Director (Principal
----------------------  Executive Officer)
Leo Berlinghieri

/s/ Ronald C. Weigner   Vice President and Chief         August 1, 2005
----------------------  Financial Officer (Principal
Ronald C. Weigner       Financial Officer and Principal
                        Accounting Officer)

/s/ John R. Bertucci    Director                         August 1, 2005
----------------------
John R. Bertucci

/s/ Robert R. Anderson  Director                         August 1, 2005
----------------------
Robert R. Anderson

/s/ James G. Berges     Director                         August 1, 2005
----------------------
James G. Berges

/s/ Richard S. Chute    Director                         August 1, 2005
----------------------
Richard S. Chute

/s/ Hans-Jochen Kahl    Director                         August 1, 2005
----------------------
Hans-Jochen Kahl

/s/ Owen W. Robbins     Director                         August 1, 2005
----------------------
Owen W. Robbins

/s/ Louis P. Valente    Director                         August 1, 2005
----------------------
Louis P. Valente


                                     - 6 -


                                  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 Wilmer Cutler Pickering Hale and Dorr LLP.

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

 23.2     Consent of PricewaterhouseCoopers 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) filed with the Securities and
      Exchange Commission on November 13, 2000.

(2)   Incorporated herein by reference from the Registrant's Quarterly Report on
      Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2001.

(3)   Incorporated herein by reference from the Registrant's Quarterly Report on
      Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2002.

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

                                     - 7 -