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                                                                    Exhibit 10.1



                               FIRST AMENDMENT TO

                 AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN

                                       OF

                              MKS INSTRUMENTS, INC.

     The Amended and Restated 1995 Stock Incentive Plan (the "Plan") be and
hereby is amended by deleting the last sentence of Section 3(b) thereof in its
entirety and inserting in lieu thereof the following:

     "The Chief Executive Officer of the Company may grant Awards to
non-executive officer employees of the Company in amounts not to exceed 500,000
shares in the aggregate or 35,000 shares to any one employee."

Adopted by the Compensation Committee of the Board of Directors on October 18,
1999.


                               SECOND AMENDMENT TO

                 AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN

                                       OF

                              MKS INSTRUMENTS, INC.

     The Amended and Restated 1995 Stock Incentive Plan (the "Plan") be and
hereby is amended by deleting the first sentence of Section 4(a) thereof in its
entirety and inserting in lieu thereof the following:

     "Beginning July 1, 2000 and subject to adjustment under Section 4(c), the
number of shares of Common Stock (as defined below in Section 4(d)) available
for Awards under the Plan shall annually increase by 4% of the total shares of
the Company's outstanding Common Stock on July 1 of each year. Such annual
increase shall occur until such time as an aggregate number of shares of Common
Stock which may be issued under the Plan is 9,750,000 shares."

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Adopted by the Board of Directors on January 31, 2000.
Approved by the Stockholders on May 17, 2000.


                               THIRD AMENDMENT TO

                 AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN

                                       OF

                              MKS INSTRUMENTS, INC.

     The Amended and Restated 1995 Stock Incentive Plan (the "Plan") be and
hereby is amended as follows:

     (1) by deleting the first sentence of Section 4(a) thereof in its entirety
     and inserting in lieu thereof the following:

     "Beginning July 1, 2001 and subject to adjustment under Section 4(c), the
number of shares of Common Stock (as defined below in Section 4(d)) available
for Awards under the Plan shall annually increase by 5% of the total shares of
the Company's outstanding Common Stock on July 1 of each year. Such annual
increase shall occur until such time as an aggregate number of shares of Common
Stock which may be issued under the Plan is 9,750,000 shares."

     (2) by deleting the last sentence of Section 3(b) thereof in its entirety
     and inserting in lieu thereof the following:

     "The Chief Executive Officer of the Company may grant Awards to
non-executive officer employees of the Company in amounts not to exceed 35,000
shares to any one employee."

Adopted by the Board of Directors on April 19, 2001.


                              MKS INSTRUMENTS, INC.

                 AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN

1.   PURPOSE

The purpose of this Amended and Restated 1995 Stock Incentive Plan (the "Plan")
of MKS Instruments, Inc., a Massachusetts corporation (the "Company"), is to
advance the interests of the Company's stockholders by enhancing the Company's
ability to attract, retain and motivate


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persons who make (or are expected to make) important contributions to the
Company by providing such persons with equity ownership opportunities and
performance-based incentives and thereby better aligning the interests of such
persons with those of the Company's stockholders. Except where the context
otherwise requires, the term "Company" shall include any present or future
subsidiary corporations of MKS Instruments, Inc. as defined in Section 424(f) of
the Internal Revenue Code of 1986, as amended, and any regulations promulgated
thereunder (the "Code").

2.   ELIGIBILITY

All of the Company's employees, officers, directors, consultants and advisors
are eligible to be granted options, restricted stock, or other stock-based
awards (each, an "Award") under the Plan. Any person who has been granted an
Award under the Plan shall be deemed a "Participant."

3.   ADMINISTRATION, DELEGATION

     a.   ADMINISTRATION BY BOARD OF DIRECTORS. The Plan will be administered by
the Board of Directors of the Company (the "Board"). The Board shall have
authority to grant Awards and to adopt, amend and repeal such administrative
rules, guidelines and practices relating to the Plan as it shall deem advisable.
The Board may correct any defect, supply any omission or reconcile any
inconsistency in the Plan or any Award in the manner and to the extent it shall
deem expedient to carry the Plan into effect and it shall be the sole and final
judge of such expediency. All decisions by the Board shall be made in the
Board's sole discretion and shall be final and binding on all persons having or
claiming any interest in the Plan or in any Award. No director or person acting
pursuant to the authority delegated by the Board shall be liable for any action
or determination relating to or under the Plan made in good faith.

     b.   DELEGATION TO EXECUTIVE OFFICERS. To the extent permitted by
applicable law, the Board may delegate to one or more executive officers of the
Company the power to make Awards and exercise such other powers under the Plan
as the Board may determine, provided that the Board shall fix the maximum number
of shares subject to Awards and the maximum number of shares for any one
Participant to be made by such executive officers. The Chief Executive Officer
of the Company may grant Awards to non-executive officer employees of the
Company in amounts not to exceed 300,000 shares in the aggregate or 20,000
shares to any one employee.

     c.   APPOINTMENT OF COMMITTEES. To the extent permitted by applicable law,
the Board may delegate any or all of its powers under the Plan to one or more
committees or subcommittees of the Board (a "Committee"). If and when the Common
Stock (as defined below in Section 4(d)) is registered under the Securities
Exchange Act of 1934 (the "Exchange Act"), the Board shall appoint one such
Committee of not less than two members, each member of which shall be an
"outside director" within the meaning of Section 162(m) of the Code and a
"non-employee director" as defined in Rule 16b-3 promulgated under the Exchange
Act." All references in the Plan to the "Board" shall mean the Board or a
Committee of the Board or the executive officer referred to in Section 3(b) to
the extent that the Board's powers or authority under the Plan have been
delegated to such Committee or executive officer.
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4.   STOCK AVAILABLE FOR AWARDS

     a.   NUMBER OF SHARES. Subject to adjustment under Section 4(c), Awards may
be made under the Plan for up to 2,500,000 shares of Common Stock (as defined
below in Section 4 (d)). If any Award expires or is terminated, surrendered or
canceled without having been fully exercised or is forfeited in whole or in part
or results in any Common Stock not being issued, the unused Common Stock (as
defined below in Section 4(d)) covered by such Award shall again be available
for the grant of Awards under the Plan, subject, however, in the case of
Incentive Stock Options (as hereinafter defined), to any limitation required
under the Code. Shares issued under the Plan may consist in whole or in part of
authorized but unissued shares or treasury shares. All share amounts set forth
in this plan reflect the 2,110-for-1 stock split approved by the Board of
Directors of the Company on December 31, 1997 (the "1997 Stock Split").

     b.   PER-PARTICIPANT LIMIT. Subject to adjustment under Section 4(c), for
Awards granted after the Common Stock (as defined below in Section 4(d)) is
registered under the Exchange Act, the maximum number of shares with respect to
which an Award may be granted to any Participant under the Plan shall be 900,000
per calendar year. The per-participant limit described in this Section 4(b)
shall be construed and applied consistently with Section 162(m) of the Code.

     c.   ADJUSTMENT TO COMMON STOCK. In the event, at any time after the 1997
Stock Split, of any stock split, stock dividend, recapitalization,
reorganization, merger, consolidation, combination, exchange of shares,
liquidation, spin-off or other similar change in capitalization or event, or any
distribution to holders of Common Stock (as defined below in Section 4(d)) other
than a normal cash dividend, (i) the number and class of securities available
under this Plan, (ii) the number and class of security and exercise price per
share subject to each outstanding Option, (iii) the repurchase price per
security subject to each outstanding Restricted Stock Award and (iv) the terms
of each other outstanding stock-based Award shall be appropriately adjusted by
the Company (or substituted Awards may be made, if applicable) to the extent the
Board shall determine, in good faith, that such an adjustment (or substitution)
is necessary and appropriate. If this Section 4(c) applies and Section 8(e) (1)
also applies to any event, Section 8(e)(1) shall be applicable to such event and
this Section 4(c) shall not be applicable.

     d.   DEFINITION OF COMMON STOCK. "Common Stock" means (i) prior to the
closing of the Company's initial public offering of common stock pursuant to an
effective registration statement under the Securities Act of 1933 ("IPO"), the
Class B Common Stock, no par value per share, of the Company, and (ii) from and
after the closing of the IPO, the Common Stock, no par value per share, of the
Company.

5.   STOCK OPTIONS

     a.   GENERAL. The Board may grant options to purchase Common Stock (each,
an "Option") and determine the number of shares of Common Stock to be covered by
each Option, the exercise price of each Option and the conditions and
limitations applicable to the exercise of

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each Option, including conditions relating to applicable federal or state
securities laws, as it considers necessary or advisable. An Option which is not
intended to be an Incentive Stock Option (as hereinafter defined) shall be
designated a "Nonstatutory Stock Option."

     b.   INCENTIVE STOCK OPTIONS. An Option that the Board intends to be an
"incentive stock option" as defined in Section 422 of the Code (an "Incentive
Stock Option") shall only be granted to employees of the Company and shall be
subject to and shall be construed consistently with the requirements of Section
422 of the Code. The Company shall have no liability to a Participant, or any
other party, if an Option (or any part thereof) which is intended to be an
Incentive Stock Option is not an Incentive Stock Option.

     c.   EXERCISE PRICE. The Board shall establish the exercise price at the
time each Option is granted and specify it in the applicable option agreement.

     d.   DURATION OF OPTIONS. Each Option shall be exercisable at such times
and subject to such terms and conditions as the Board may specify in the
applicable option agreement. No Option will be granted for a term in excess of
10 years.

     e.   EXERCISE OF OPTION. Options may be exercised only by delivery to the
Company of a written notice of exercise signed by the proper person together
with payment in full as specified in Section 5(f) for the number of shares for
which the Option is exercised.

     f.   PAYMENT UPON EXERCISE. Common Stock purchased upon the exercise of an
Option granted under the Plan shall be paid for as follows:

          i.   in cash or by check, payable to the order of the Company;

          ii.  except as the Board may otherwise provide in an Option Agreement,
delivery of an irrevocable and unconditional undertaking by a creditworthy
broker to deliver promptly to the Company sufficient funds to pay the exercise
price, delivery by the Participant to the Company of a copy of irrevocable and
unconditional instructions to a creditworthy broker to deliver promptly to the
Company cash or a check sufficient to pay the exercise price, or by delivery of
shares of Common Stock owned by the Participant valued at the fair market value
as determined by the Board in good faith ("Fair Market Value"), which Common
Stock was owned by the Participant at least six months prior to such delivery;

          iii. to the extent permitted by the Board and explicitly provided in
an Option Agreement (i) by delivery of a promissory note of the Participant to
the Company on terms determined by the Board, or (ii) by payment of such other
lawful consideration as the Board may determine; or

          iv.  any combination of the above permitted forms of payment.

6.   RESTRICTED STOCK

     a.   GRANTS. The Board may grant Awards entitling recipients to acquire
shares of Common Stock, subject to the right of the Company to repurchase all or
part of such shares at
<PAGE>

their issue price or other stated or formula price (or to require forfeiture of
such shares if issued at no cost) from the recipient in the event that
conditions specified by the Board in the applicable Award are not satisfied
prior to the end of the applicable restriction period or periods established by
the Board for such Award (each, "Restricted Stock Award").

     b.   TERMS AND CONDITIONS. The Board shall determine the terms and
conditions of any such Restricted Stock Award, including the conditions for
repurchase (or forfeiture) and the issue price, if any. Any stock certificates
issued in respect of a Restricted Stock Award shall be registered in the name of
the Participant and, unless otherwise determined by the Board, deposited by the
Participant, together with a stock power endorsed in blank, with the Company (or
its designee). At the expiration of the applicable restriction periods, the
Company (or such designee) shall deliver the certificates no longer subject to
such restrictions to the Participant or if the Participant has died, to the
beneficiary designated, in a manner determined by the Board, by a Participant to
receive amounts due or exercise rights of the Participant in the event of the
Participant's death (the "Designated Beneficiary"). In the absence of an
effective designation by a Participant, Designated Beneficiary shall mean the
Participant's estate.

7.   OTHER STOCK-BASED AWARDS

The Board shall have the right to grant other Awards based upon the Common
Stock, having such terms and conditions as the Board may determine including the
grant of shares based upon certain conditions, the grant of securities
convertible into Common Stock and the grant of stock appreciation rights.

8.   GENERAL PROVISIONS APPLICABLE TO AWARDS

     a.   TRANSFERABILITY OF AWARDS. Except as the Board may otherwise determine
or provide in an Award, Awards shall not be sold, assigned, transferred, pledged
or otherwise encumbered by the person to whom they are granted, either
voluntarily or by operation of law, except by will or the laws of descent and
distribution, and, during the life of the Participant, shall be exercisable only
by the Participant. References to a Participant, to the extent relevant in the
context, shall include references to authorized transferees.

     b.   DOCUMENTATION. Each Award under the Plan shall be evidenced by a
written instrument in such form as the Board shall determine. Each Award may
contain terms and conditions in addition to those set forth in the Plan.

     c.   BOARD DISCRETION. Except as otherwise provided by the Plan, each type
of Award may be made alone or in addition or in relation to any other type of
Award. The terms of each type of Award need not be identical, and the Board need
not treat Participants uniformly.

     d.   TERMINATION OF STATUS. The Board shall determine the effect on an
Award of the disability, death, retirement, authorized leave of absence or other
change in the employment or other status of a Participant and the extent to
which, and the period during which, the Participant, the Participant's legal
representative, conservator, guardian or Designated Beneficiary may exercise
rights under the Award.



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     e.   ACQUISITION EVENTS

          (1)  CONSEQUENCES OF ACQUISITION EVENTS. Upon the occurrence of an
Acquisition Event (as defined below), or the execution by the Company of any
agreement with respect to an Acquisition Event, the Board shall take any one or
more of the following actions with respect to then outstanding Awards: (i)
provide that outstanding Options shall be assumed, or equivalent Options shall
be substituted, by the acquiring or succeeding corporation (or an affiliate
thereof), provided that any such Options substituted for Incentive Stock Options
shall satisfy, in the determination of the Board, the requirements of Section
424(a) of the Code; (ii) if the acquisition or succeeding corporation refuses or
is unable to assume outstanding Options or grant Options in substitution
therefor pursuant to clause (i), upon written notice to the Participants,
provide that all then unexercised Options will become exercisable in full as of
a specified time (the "Acceleration Time") prior to the Acquisition Event and
will terminate immediately prior to the consummation of such Acquisition Event,
except to the extent exercised by the Participants between the Acceleration Time
and the consummation of such Acquisition Event; (iii) in the event of an
Acquisition Event under the terms of which holders of Common Stock will receive
upon consummation thereof a cash payment for each share of Common Stock
surrendered pursuant to such Acquisition Event (the "Acquisition Price"),
provide that all outstanding Options shall terminate upon consummation of such
Acquisition Event and each Participant shall receive, in exchange therefor, a
cash payment equal to the amount (if any) by which (A) the Acquisition Price
multiplied by the number of shares of Common Stock subject to such outstanding
Options (whether or not then exercisable), exceeds (B) the aggregate exercise
price of such Options; (iv) provide that all Restricted Stock Awards then
outstanding shall become free of all restrictions prior to the consummation of
the Acquisition Event; and (v) provide that any other stock-based Awards
outstanding (A) shall become exercisable, realizable or vested in full, or shall
be free of all conditions or restrictions, as applicable to each such Award,
prior to the consummation of the Acquisition Event, or (B), if applicable, shall
be assumed, or equivalent Awards shall be substituted, by the acquiring or
succeeding corporation (or an affiliate thereof).

          An "Acquisition Event" shall mean: (a) any merger or consolidation
which results in the voting securities of the Company outstanding immediately
prior thereto representing immediately thereafter (either by remaining
outstanding or by being converted into voting securities of the surviving or
acquiring entity) less than 50% of the combined voting power of the voting
securities of the Company or such surviving or acquiring entity outstanding
immediately after such merger or consolidation; (b) any sale of all or
substantially all of the assets of the Company; or (c) the complete liquidation
of the Company.

          (2)  ASSUMPTION OF OPTIONS UPON CERTAIN EVENTS. The Board may grant
Awards under the Plan in substitution for stock and stock-based awards held by
employees of another corporation who become employees of the Company as a result
of a merger or consolidation of the employing corporation with the Company or
the acquisition by the Company of property or stock of the employing
corporation. The substitute Awards shall be granted on such terms and conditions
as the Board considers appropriate in the circumstances.
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     f.   WITHHOLDING. Each Participant shall pay to the Company, or make
provision satisfactory to the Board for payment of, any taxes required by law to
be withheld in connection with Awards to such Participant no later than the date
of the event creating the tax liability. The Board may allow Participants to
satisfy such tax obligations in whole or in part in shares of Common Stock,
including shares retained from the Award creating the tax obligation, valued at
their Fair Market Value. The Company may, to the extent permitted by law, deduct
any such tax obligations from any payment of any kind otherwise due to a
Participant.

     g.   AMENDMENT OF AWARD. The Board may amend, modify or terminate any
outstanding Award, including but not limited to, substituting therefor another
Award of the same or a different type, changing the date of exercise or
realization, and converting an Incentive Stock Option to a Nonstatutory Stock
Option, provided that the Participant's consent to such action shall be required
unless the Board determines that the action, taking into account any related
action, would not materially and adversely affect the Participant.

     h.   CONDITIONS ON DELIVERY OF STOCK. The Company will not be obligated to
deliver any shares of Common Stock pursuant to the Plan or to remove
restrictions from shares previously delivered under the Plan until (i) all
conditions of the Award have been met or removed to the satisfaction of the
Company, (ii) in the opinion of the Company's counsel, all other legal matters
in connection with the issuance and delivery of such shares have been satisfied,
including any applicable securities laws and any applicable stock exchange or
stock market rules and regulations, and (iii) the Participant has executed and
delivered to the Company such representations or agreements as the Company may
consider appropriate to satisfy the requirements of any applicable laws, rules
or regulations.

     i.   ACCELERATION. The Board may at any time provide that any Options shall
become immediately exercisable in full or in part, that any Restricted Stock
Awards shall be free of all restrictions or that any other stock-based Awards
may become exercisable in full or in part or free of some or all restrictions or
conditions, or otherwise realizable in full or in part, as the case may be.

9.   MISCELLANEOUS

     a.   NO RIGHT TO EMPLOYMENT OR OTHER STATUS. No person shall have any claim
or right to be granted an Award, and the grant of an Award shall not be
construed as giving a Participant the right to continued employment or any other
relationship with the Company. The Company expressly reserves the right at any
time to dismiss or otherwise terminate its relationship with a Participant free
from any liability or claim under the Plan, except as expressly provided in the
applicable Award.

     b.   NO RIGHTS AS STOCKHOLDER. Subject to the provisions of the applicable
Award, no Participant or Designated Beneficiary shall have any rights as a
stockholder with respect to any shares of Common Stock to be distributed with
respect to an Award until becoming the record holder of such shares.
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     c.   EFFECTIVE DATE AND TERM OF PLAN. The Plan shall become effective on
the date on which it is adopted by the Board, but no Award granted to a
Participant designated as subject to Section 162(m) by the Board shall become
exercisable, vested or realizable, as applicable to such Award, unless and until
the Plan has been approved by the Company's stockholders. No Awards shall be
granted under the Plan after the completion of ten years from the earlier of (i)
the date on which the Plan was adopted by the Board or (ii) the date the Plan
was approved by the Company's stockholders, but Awards previously granted may
extend beyond that date.

     d.   AMENDMENT OF PLAN. The Board may amend, suspend or terminate the Plan
or any portion thereof at any time, provided that no Award granted to a
Participant designated as subject to Section 162(m) by the Board after the date
of such amendment shall become exercisable, realizable or vested, as applicable
to such Award (to the extent that such amendment to the Plan was required to
grant such Award to a particular Participant), unless and until such amendment
shall have been approved by the Company's stockholders.

     e.   STOCKHOLDER APPROVAL. For purposes of this Plan, stockholder approval
shall mean approval by a vote of the stockholders in accordance with the
requirements of Section 162(m) of the Code.

     f.   GOVERNING LAW. The provisions of the Plan and all Awards made
hereunder shall be governed by and interpreted in accordance with the laws of
The Commonwealth of Massachusetts, without regard to any applicable conflicts of
law rules or provisions.

                                         Adopted by the Board of Directors on
                                         November 30, 1995

                                         Adopted by the Stockholders on
                                         May 17, 1996

                                         Approved as Amended and Restated by the
                                         Board of Directors on December 31, 1997

                                         Approved as Amended and Restated by the
                                         Stockholders on January 9, 1998