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                                                                    Exhibit 10.6

                               FIRST AMENDMENT TO

             AMENDED AND RESTATED 1999 EMPLOYEE STOCK PURCHASE PLAN

                                       OF

                              MKS INSTRUMENTS, INC.

     Whereas, the above titled plan (the "Plan") was adopted by the Board of
Directors (the "Board") of MKS Instruments, Inc. (the "Company") and

     Whereas, Section 17 of the Plan provides that the Board may at any time,
and from time to time, amend this Plan in any respect,

     Now therefore, the Plan is hereby amended, effective for the Plan Period
beginning June 1, 2001, as follows:

          Section 2(b) is amended by deleting the requirement that a person be
     employed by the Company or a Designated Subsidiary for "at least six (6)
     months prior to enrolling in the Plan" and substituting a requirement that
     a person be employed for "at least three (3) months prior to enrolling in
     the Plan".

     Now therefore, the Plan is hereby further amended, effective for the Plan
Period beginning June 1, 2001, as follows:

     The first paragraph of Section 9 is hereby deleted in its entirety and the
following paragraph is inserted in lieu thereof:

          "9. PURCHASE OF SHARES. On the Offering Commencement Date of each Plan
     Period, the Company will grant to each eligible employee who is then a
     participant in the Plan an option ("Option") to purchase on the last
     business day of such Plan Period (the "Exercise Date"), at the Option Price
     hereinafter provided for, the largest number of shares (fractional or
     whole) of Common Stock of the Company as does not exceed the number of
     shares determined by multiplying $2,083 by the number of full months in the
     Offering Period and dividing the results by the closing price (as defined
     below) on the Offering Commencement Date of such Plan Period."


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



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                             MKS INSTRUMENTS, INC.

             AMENDED AND RESTATED 1999 EMPLOYEE STOCK PURCHASE PLAN

     The purpose of this Plan is to provide eligible employees of MKS
Instruments, Inc. (the "Company") and certain of its subsidiaries with
opportunities to purchase shares of the Company's Common Stock, no par value per
share (the "Common Stock"), commencing on June 1, 1999; provided, that at such
time the Company's Common Stock shall be listed for trading on the Nasdaq
National Market or a national securities exchange. Four hundred fifty thousand
(450,000) shares of Common Stock in the aggregate have been approved for this
purpose. This Plan is intended to qualify as an "employee stock purchase plan"
as defined in Section 423 of the Internal Revenue Code of 1986, as amended (the
"Code") and the regulations promulgated thereunder, and shall be interpreted
consistent therewith. All share amounts set forth in this Plan reflect the
3-for-2 stock split approved by the Board of Directors of the Company on
February 10, 1999 (the "1999 Stock Split").

1.   ADMINISTRATION. The Plan will be administered by the Company's Board of
     Directors (the "Board") or by a Committee appointed by the Board (the
     "Committee"). The Board or the Committee has authority to make rules and
     regulations for the administration of the Plan and its interpretation and
     decisions with regard thereto shall be final and conclusive.

2.   ELIGIBILITY. All employees of the Company, including Directors who are
     employees, and all employees of any subsidiary of the Company (as defined
     in Section 424(f) of the Code) designated by the Board or the Committee
     from time to time (a "Designated Subsidiary"), including employees of the
     Company or any designated Subsidiary who are "highly compensated" within
     the meaning of Section 414(q) of the Code, are eligible to participate in
     any one or more of the Offerings (as defined in Section 9) to purchase
     Common Stock under the Plan provided that:

          (a)  they are customarily employed by the Company or a Designated
               Subsidiary for more than 20 hours a week and for more than five
               months in a calendar year; and

          (b)  they have been employed by the Company or a Designated Subsidiary
               for at least six months prior to enrolling in the Plan; and

          (c)  they are employees of the Company or a Designated Subsidiary on
               the first day of the applicable Plan Period (as defined below).

         No employee may be granted an option hereunder if such employee,
immediately after the option is granted, owns 5% or more of the total combined
voting power or value of the stock of the Company or any subsidiary. For
purposes of the preceding sentence, the attribution rules of




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Section 424(d) of the Code shall apply in determining the stock ownership of an
employee, and all stock which the employee has a contractual right to purchase
shall be treated as stock owned by the employee.


3.   OFFERINGS. The Company will make one or more offerings ("Offerings") to
employees to purchase stock under this Plan. Offerings will begin each June 1
and December 1, or the first business day thereafter (the "Offering Commencement
Dates"). Each Offering Commencement Date will begin a six (6) month period (a
"Plan Period") during which Payroll deductions will be made and held for the
purchase of Common Stock at the end of the Plan Period. The Board or the
Committee may, at its discretion, choose a different Plan Period of twelve (12)
months or less for subsequent Offerings.

4.   PARTICIPATION. An employee eligible on the Offering Commencement Date of
any Offering may participate in such Offering by completing and forwarding a
payroll deduction authorization form to the employee's appropriate payroll
office at least 30 days prior to the applicable Offering Commencement Date. The
form will authorize a regular payroll deduction from the Compensation, as
defined below, received by the employee during the Plan Period. Unless an
employee files a new form or withdraws from the Plan, his deductions and
purchases will continue at the same rate for future Offerings under the Plan as
long as the Plan remains in effect. The term "Compensation" means the amount of
money reportable on the employee's Federal Income Tax Withholding Statement,
including overtime, shift premium, incentive or bonus awards and sales expenses
and excluding allowances and reimbursements for expenses such as relocation
allowances for travel expenses, income or gains on the exercise of Company stock
options or stock appreciation rights, and similar items, whether or not shown on
the employee's Federal Income Tax Withholding Statement, but including, in the
case of salespersons, sales commissions to the extent determined by the Board or
the Committee.

5.   DEDUCTIONS. The Company will maintain payroll deduction accounts for all
participating employees. With respect to any Offering made under this Plan, an
employee may authorize a payroll deduction in any whole percent amount up to a
maximum of 10% (or such lower percentage as may be established by the Board or
the Committee) of the Compensation he or she receives during the Plan Period or
such shorter period during which deductions from payroll are made. The minimum
payroll deduction is such percentage of compensation as may be established from
time to time by the Board or the Committee.

     No employee may be granted an Option (as defined in Section 9) which
permits his rights to purchase Common Stock under this Plan and any other
employee stock purchase plan (as defined in Section 423(b) of the Code) of the
Company and its subsidiaries, to accrue at a rate which exceeds $25,000 of the
fair market value of such Common Stock (determined at the Offering Commencement
Date of the Plan Period) for each calendar year in which the Option is
outstanding at any time.



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6.   DEDUCTION CHANGES. An employee may decrease, subject to section 5 hereof or
discontinue his payroll deduction once during any Plan Period, by filing a new
payroll deduction authorization form. However, an employee may not elect to
increase his payroll deduction during a Plan Period. If an employee elects to
discontinue his payroll deductions during a Plan Period, but does not elect to
withdraw his funds pursuant to Section 8 hereof, funds deducted prior to his
election to discontinue will be applied to the purchase of Common Stock on the
Exercise Date (as defined below).

7.   INTEREST. Interest will not be paid on employee accounts.

8.   WITHDRAWAL OF FUNDS. An employee may at any time prior to the close of
business on the last business day in a Plan Period and for any reason
permanently draw out the balance accumulated in the employee's account and
thereby withdraw from participation in an Offering. Partial withdrawals are not
permitted. The employee may not begin participation again during the remainder
of the Plan Period. The employee may participate in any subsequent Offering in
accordance with terms and conditions established by the Board or the Committee.

9.   PURCHASE OF SHARES. On the Offering Commencement Date of each Plan Period,
the Company will grant to each eligible employee who is then a participant in
the Plan an option ("Option") to purchase on the last business day of such Plan
Period (the "Exercise Date"), at the Option Price hereinafter provided for, the
largest number of whole shares of Common Stock of the Company as does not exceed
the number of shares determined by multiplying $2,083 by the number of full
months in the Offering Period and dividing the results by the closing price (as
defined below) on the Offering Commencement Date of such Plan Period.

     The purchase price for each share purchased will be 85% of the closing
price of the Common Stock on (i) the first business day of such Plan Period or
(ii) the Exercise Date, whichever closing price shall be less. Such closing
price shall be (a) the closing price on any national securities exchange on
which the Common Stock is listed, (b) the closing price of the Common Stock on
the Nasdaq National Market or (c) the average of the closing bid and asked
prices in the over-the-counter-market, whichever is applicable, as published in
The Wall Street Journal. If no sales of Common Stock were made on such a day,
the price of the Common Stock for purposes of clauses (a) and (b) above shall be
the reported price for the next preceding day on which sales were made.

     Each employee who continues to be a participant in the Plan on the Exercise
Date shall be deemed to have exercised his Option at the Option Price on such
date and shall be deemed to have purchased from the Company the number of full
shares of Common Stock reserved for the purpose of the Plan that his accumulated
payroll deductions on such date will pay for, but not in excess of the maximum
number determined in the manner set forth above.




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     Any balance remaining in an employee's payroll deduction account at the end
of a Plan Period will be automatically refunded to the employee, except that any
balance which is less than the purchase price of one share of Common Stock will
be carried forward into the employee's payroll deduction account for the
following Offering, unless the employee elects not to participate in the
following Offering under the Plan, in which case the balance in the employee's
account shall be refunded.

10.  ISSUANCE OF CERTIFICATES. Certificates representing shares of Common Stock
purchased under the Plan may be issued only in the name of the employee, in the
name of the employee and another person of legal age as joint tenants with
rights of survivorship, or (in the Company's sole discretion) in the name of a
brokerage firm, bank or other nominee holder designated by the employee. The
Company may, in its sole discretion and in compliance with applicable laws,
authorize the use of book entry registration of shares in lieu of issuing stock
certificates.

11.  RIGHTS ON RETIREMENT, DEATH OR TERMINATION OF EMPLOYMENT. In the event of a
participating employee's termination of employment prior to the last business
day of a Plan Period, no payroll deduction shall be taken from any pay due and
owing to an employee and the balance in the employee's account shall be paid to
the employee or, in the event of the employee's death, (a) to a beneficiary
previously designated in a revocable notice signed by the employee (with any
spousal consent required under state law) or (b) in the absence such a
designated beneficiary, to the executor or administrator of the employee's
estate or (c) if no such executor or administrator has been appointed to the
knowledge of the Company, to such other person(s) as the Company may, in its
discretion, designate. If, prior to the last business day of the Plan Period,
the Designated Subsidiary by which an employee is employed shall cease to be a
subsidiary of the Company, or if the employee is transferred to a subsidiary of
the Company that is not a Designated Subsidiary, the employee shall be deemed to
have terminated employment for the purposes of this Plan.

12.  OPTIONEES NOT STOCKHOLDERS. No employee shall have any rights as a
stockholder with respect to any shares of Common Stock to be distributed with
respect to an Option until becoming the record holder or such shares.
Notwithstanding the foregoing, in the event the Company effects a split of the
Common Stock by means of a stock dividend (and the exercise price of and the
number of shares subject to such Option are adjusted as of the date of the
distribution of the dividend rather than as of the record date for such
dividend), then an optionee who is deemed to have exercised an Option between
the record date and the distribution date for such stock dividend shall be
entitled to receive, on the distribution date, the stock dividend with respect
to the shares of Common Stock.





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13.  RIGHTS NOT TRANSFERABLE. Rights under this Plan are not transferable by a
participating employee other than by will or the laws of descent and
distribution, and are exercisable during the employee's lifetime only by the
employee.

14.  APPLICATION OF FUNDS. All funds received or held by the Company under this
Plan may be combined with other corporate funds and may be used for any
corporate purpose.

15.  ADJUSTMENT IN CASE OF CHANGES AFFECTING COMMON STOCK. In the event, at any
time after the 1999 Stock Split, of a subdivision of outstanding shares of
Common Stock, or the payment of a dividend in Common Stock, the number of shares
approved for this Plan, and the share limitation set forth in Section 9, shall
be increased proportionately, and such other adjustment shall be made as may be
deemed equitable by the Board or the Committee. In the event of any other change
affecting the Common Stock, such adjustment shall be made as may be deemed
equitable by the Board or the Committee to give proper effect to such event.

16.  MERGER. If the Company shall at any time merge or consolidate with another
corporation and the holders of the capital stock of the Company immediately
prior to such merger or consolidation continue to hold at least 80% by voting
power of the capital stock of the surviving corporation ("Continuity of
Control"), the holder of each Option then outstanding will thereafter be
entitled to receive at the next Exercise Date upon the exercise of such Option
for each share as to which such Option shall be exercised the securities or
property which a holder of one share of the Common Stock was entitled to upon
and at the time of such merger or consolidation, and the Board or the Committee
shall take such steps in connection with such merger or consolidation as the
Board or the Committee shall deem necessary to assure that the provisions of
Section 15 shall thereafter be applicable, as nearly as reasonably may be, in
relation to the said securities or property as to which such holder of such
Option might thereafter be entitled to receive thereunder.

     In the event of a merger or consolidation of the Company with or into
another corporation which does not involve Continuity of Control, or of a sale
of all or substantially all of the assets of the Company while unexercised
Options remain outstanding under the Plan, all outstanding Options shall be
cancelled by the Board or the Committee as of the effective date of any such
transaction, provided that notice of such cancellation shall be given to each
holder of an Option, and each holder of an Option shall have the right to
exercise such Option in full based on payroll deductions then credited to his
account as of a date determined by the Board or the Committee, which date shall
not be less than ten (10) days preceding the effective date of such transaction.

17.  AMENDMENT OF THE PLAN. The Board may at any time, and from time to time,
amend this Plan in any respect, except that (a) if the approval of any such
amendment by



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the shareholders of the Company is required by Section 423 of the Code, such
amendment shall not be effected without such approval, and (b) in no event may
any amendment be made which would cause the Plan to fail to comply with Section
423 of the Code.

18.  SUFFICIENT SHARES. In the event that the total number of shares of Common
Stock specified in elections to be purchased under any Offering plus the number
of shares purchased under previous Offerings under this Plan exceeds the maximum
number of shares issuable under this Plan, the Board or the Committee will allot
the shares then available on a pro rata basis.

19.  TERMINATION OF THE PLAN. This Plan may be terminated at any time by the
Board. Upon termination of this Plan all amounts in the accounts of
participating employees shall be promptly refunded.

20.  GOVERNMENTAL REGULATIONS. The Company's obligation to sell and deliver
Common Stock under this Plan is subject to listing on a national stock exchange
or quotation on the Nasdaq National Market and the approval of all governmental
authorities required in connection with the authorization, issuance or sale of
such stock.

21.  GOVERNING LAW. The Plan shall be governed by Massachusetts law except to
the extent that such law is preempted by federal law.

22.  ISSUANCE OF SHARES. Shares may be issued upon exercise of an Option from
authorized but unissued Common Stock, from shares held in the treasury of the
Company, or from any other proper source.

23.  NOTIFICATION UPON SALE OF SHARES. Each employee agrees, by entering the
Plan, to promptly give the Company notice of any disposition of shares purchased
under the Plan where such disposition occurs within two years after the date of
grant of the Option pursuant to which such shares were purchased or one year
after the date of exercise of the Option.

24.  EFFECTIVE DATE AND APPROVAL OF STOCKHOLDERS. The Plan shall take effect on
June 1, 1999 if at such time the Common Stock is listed for trading on the
Nasdaq National Market or a national securities exchange, subject to approval by
the stockholders of the Company as required by Section 423 of the Code, which
approval must occur within twelve months of the adoption of the Plan by the
Board.


                                        Adopted by the Board of Directors
                                        on February 10, 1999

                                        Approval by the Stockholders
                                        on February 17, 1999

                                        Amended and Restated by the Board
                                        of Directors on April 22, 1999




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