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                                                                   Exhibit 10.49

                           CHANGE IN CONTROL AGREEMENT

     AGREEMENT dated as of May 9, 2002 by and between Aspen Technology, Inc., a
Delaware corporation (the "Company"), and Wayne Sim (the "Executive").

     WHEREAS, the Company considers it essential to the best interests of its
stockholders to foster the continuous employment of key management personnel;
and

     WHEREAS, the Company has determined that appropriate steps should be taken
to reinforce and encourage the continued attention and dedication of members of
the Company's management, including the Executive, to their assigned duties with
the Company without distraction in the face of potentially disturbing
circumstances arising from the possibility of a Change in Control (as defined
herein);

     NOW THEREFORE, in consideration of the premises and the mutual covenants
herein contained, and for other valuable consideration, the Company and the
Executive hereby agree as follows:

     1. DEFINED TERMS. The definitions of capitalized terms used in this
Agreement are provided in the last section hereof.

     2. TERM OF AGREEMENT. This Agreement shall commence on the date hereof and
shall continue in effect through June 30, 2003. Thereafter, this Agreement shall
be automatically renewed for successive one year terms unless the Company sends
written notice of termination to the Executive at least 60 days before the
expiration date of this Agreement, which termination will be effective at that
expiration date. If a Change in Control shall have occurred during the term of
this Agreement, however, this Agreement shall continue in effect for a period of
three years beyond the last day of the month in which the Change in Control
occurred. Notwithstanding the foregoing provisions of this Section 2, this
Agreement shall terminate, unless earlier terminated in accordance with this
Agreement, (i) one year after the Executive is notified in accordance with
Section 9 hereof that the Compensation Committee, upon recommendation of the
Company's chief executive officer, has voted to terminate this Agreement or (ii)
if earlier, immediately after the Executive is notified in accordance with
Section 9 hereof that the Compensation Committee has determined that the
Executive's level of responsibility (other than

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reporting responsibility) has substantially changed from the Executive's current
level of responsibility, in either case only if the notification occurs prior to
a Potential Change in Control that results in a Change in Control.

     3. PAYMENTS AFTER CHANGE IN CONTROL.

     3.1 If the Executive's employment shall be terminated for any reason
following a Change in Control and during the term of this Agreement, the Company
shall pay the Executive's full salary to the Executive through the Date of
Termination at the rate in effect at the time the Notice of Termination is
given, together with all compensation and benefits payable to the Executive
through the Date of Termination under the terms of any compensation or benefit
plan, program or arrangement maintained by the Company during such period.

     3.2 Subject to Section 3.3, the Company shall pay to the Executive the
payments described in this Section 3.2 (the "Severance Payments") upon the
termination of the Executive's employment following a Change in Control and
during the term of this Agreement, in addition to the payments and benefits
described in Section 3.1, unless such termination is (i) by the Company for
Cause, (ii) by reason of death, (iii) by the Executive without Good Reason, or
(iv) after the Executive shall have attained age 70. In lieu of any further
salary payments to the Executive for periods subsequent to the Date of
Termination and in lieu of any severance benefits otherwise payable to the
Executive under any then existing broad-based employee severance plan, the
Company shall pay to the Executive a lump sum severance payment, in cash, equal
to three times the sum of (x) the higher of the Executive's annual base salary
in effect immediately prior to the occurrence of the event or circumstance upon
which the Notice of Termination is based or in effect immediately prior to the
Change in Control and (y) the higher of the average of the annual bonuses paid
to the Executive for the three years (or the number of years employed, if less)
immediately preceding the occurrence of the event or circumstance upon which the
Notice of Termination is based or the Change in Control. In lieu of any further
life, disability, accident and health insurance benefits otherwise due to the
Executive, the Company shall pay to the Executive a lump sum amount, in cash,
equal to the cost to the Company (as determined by the Company in good faith
with reference to its most recent actual experience) of providing such benefits,
to the extent that the Executive is eligible to receive such benefits
immediately prior to the Notice of

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Termination, for a period of three years commencing on the Date of Termination.

     3.3 The payments provided for in Section 3.2 shall be made not later than
the fifth day following the Date of Termination.

     3.4 The Company also shall pay to the Executive all legal fees and expenses
incurred by the Executive in seeking to obtain or enforce any benefit or right
provided by this Agreement, payable within five business days after delivery of
the Executive's written requests for payment accompanied with such evidence of
fees and expenses incurred as the Company reasonably may require.

     4. CERTAIN ADDITIONAL PAYMENTS BY THE COMPANY.

     4.1 Notwithstanding any other provisions of this Agreement, in the event
that any payment or benefit received or to be received by the Executive in
connection with a Change in Control or the termination of the Executive's
employment (all such payments and benefits, including the Severance Payments,
the "Total Payments") is determined to be subject (in whole or part) to the
Excise Tax, then the Executive shall be entitled to receive an additional
payment (a "Gross-Up Payment") in an amount such that after payment by the
Executive of all taxes (including any interest or penalties imposed with respect
to such taxes), including without limitation any income taxes and Excise Tax
imposed upon the Gross-Up Payment, the Executive retains an amount equal to the
Total Payments. Notwithstanding the foregoing provisions of this Section 4.1, if
it shall be determined that the Executive is entitled to a Gross-Up Payment, but
that the Total Payments do not exceed 110% of the greatest amount (the "Reduced
Amount") that could be paid to the Executive such that the receipt thereof would
not give rise to any Excise Tax, then no Gross-Up Payment shall be made to the
Executive and the Total Payments shall be reduced to the Reduced Amount.

     4.2 All determinations required to be made under this Section 4, including
whether and when a Gross-Up Payment is required and the amount of such Gross-Up
Payment and the assumptions to be utilized in arriving at such determination,
shall be made by the Company's accountants or such other certified public
accounting firm reasonably acceptable to the Company as may be designated by the
Executive (the "Accounting Firm") which shall provide detailed supporting
calculations both to the Company and the Executive.

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     5. TERMINATION PROCEDURES.

     5.1 NOTICE OF TERMINATION. After a Change in Control and during the term of
this Agreement, any purported termination of the Executive's employment (other
than by reason of death) shall be communicated by written Notice of Termination
from one party hereto to the other party hereto in accordance with Section 8.
Further, a Notice of Termination for Cause is required to include a copy of a
resolution duly adopted by the affirmative vote of not less than three-quarters
of the entire membership of the Board at a meeting of the Board which was called
and held for the purpose of considering such termination (after reasonable
notice to the Executive and an opportunity for the Executive, together with the
Executive's counsel, to be heard before the Board) finding that, in the good
faith opinion of the Board, the Executive was guilty of conduct set forth in the
definition of Cause.

     5.2 DATE OF TERMINATION. "Date of Termination", with respect to any
purported termination of the Executive's employment after a Change in Control
and during the term of this Agreement, shall mean the date specified in the
Notice of Termination (which, in the case of a termination by the Company
otherwise than for Cause, shall not be less than thirty days and, in the case of
a termination by the Executive, shall not be less than fifteen days nor more
than sixty days, respectively, from the date such Notice of Termination is
given).

     6. NO MITIGATION. If the Executive's employment by the Company is
terminated during the term of this Agreement, the Executive is not required to
seek other employment or to attempt in any way to reduce any amounts payable to
the Executive by the Company pursuant to Section 3. Further, the amount of any
payment or benefit provided for in Section 3 shall not be reduced by any
compensation earned by the Executive as the result of employment by another
employer, by retirement benefits, by offset against any amount claimed to be
owed by the Executive to the Company, or otherwise.

     7. EXECUTIVE'S COVENANTS. The Executive agrees that, subject to the terms
and conditions of this Agreement, in the event of a Potential Change in Control
during the term of this Agreement, the Executive will remain in the employ of
the Company until the earliest of (i) a date which is six months from the date
of such Potential Change of Control, (ii) the date of a Change in Control, (iii)
the date of

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termination by the Executive of the Executive's employment for Good Reason
(determined by treating the Potential Change in Control as a Change in Control
in applying the definition of Good Reason), by reason of death or Retirement; or
(iv) the termination by the Company of the Executive's employment for any
reason.

     8. SUCCESSORS; BINDING AGREEMENT.

     8.1 In addition to any obligations imposed by law upon any successor to the
Company, the Company will require any successor (whether direct or indirect, by
purchase, merger, consolidation or otherwise) to all or substantially all of the
business and/or assets of the Company to expressly assume and agree to perform
this Agreement in the same manner and to the same extent that the Company would
be required to perform it if no such succession had taken place. Failure of the
Company to obtain such assumption and agreement prior to the effectiveness of
any such succession shall be a breach of this Agreement and shall entitle the
Executive to compensation from the Company in the same amount and on the same
terms as the Executive would be entitled to hereunder if the Executive were to
terminate the Executive's employment for Good Reason after a Change in Control,
except that, for purposes of implementing the foregoing, the date on which any
such succession becomes effective shall be deemed the Date of Termination.

     8.2 This Agreement shall inure to the benefit of and be enforceable by the
Executive's personal or legal representatives. If the Executive shall die while
any amount would still be payable to the Executive hereunder (other than amounts
which, by their terms, terminate upon the death of the Executive) if the
Executive had continued to live, all such amounts, unless otherwise provided
herein, shall be paid in accordance with the terms of this Agreement to the
Executive's representatives.

     9. NOTICES. For the purpose of this Agreement, notices and all other
communications provided for in the Agreement shall be in writing and shall be
deemed to have been duly given when delivered or mailed by United States
certified or registered mail, return receipt requested, postage prepaid,
addressed to the respective addresses set forth below, or to such other address
as either party may have furnished to the other in writing in accordance
herewith, except that notice of change of address shall be effective only upon
actual receipt:

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                           To the Company:
                           Aspen Technology, Inc.
                           Ten Canal Park
                           Cambridge, MA  02141

                           Attention:  General Counsel

                           To the Executive:
                           Wayne Sim
                           35 Spring Gate Estates SW
                           Calgary, Alberta

     10. MISCELLANEOUS. No provision of this Agreement may be modified, waived
or discharged unless such waiver, modification or discharge is agreed to in
writing and signed by the Executive and such officer as may be specifically
designated by the Board. Except as expressly provided herein, no waiver by
either party hereto at any time of any breach by the other party hereto of, or
compliance with, any condition or provision of this Agreement to be performed by
such other party shall be deemed a waiver of similar or dissimilar provisions or
conditions at the same or at any prior or subsequent time. No agreements or
representations, oral or otherwise, express or implied, with respect to the
subject matter hereof have been made by either party which are not expressly set
forth in this Agreement. The validity, interpretation, construction and
performance of this Agreement shall be governed by the laws of the Commonwealth
of Massachusetts, and this Agreement shall be an instrument under seal. All
references to sections of the Exchange Act or the Code shall be deemed also to
refer to any successor provisions to such sections. Any payments provided for
hereunder shall be paid net of any applicable withholding required under
federal, state or local law and any additional withholding to which the
Executive has agreed.

     11. SETTLEMENT OF DISPUTES; ARBITRATION. All claims by the Executive for
benefits under this Agreement shall be directed to and determined by the Board
and shall be in writing. Any denial by the Board of a claim for benefits under
this Agreement shall be delivered to the Executive in writing and shall set
forth the specific reasons for the denial and the specific provisions of this
Agreement relied upon. Any further dispute or controversy arising under or in
connection with this Agreement shall be settled exclusively by arbitration in
Boston, Massachusetts, in accordance with the rules of the American Arbitration
Association then in effect. Judgment may be entered on

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the arbitrator' s award in any court having jurisdiction. The Executive shall,
however, be entitled to seek specific performance of the Executive's right to be
paid until the Date of Termination during the pendency of any dispute or
controversy arising under or in connection with this Agreement.

     12. DEFINITIONS. For purposes of this Agreement, the following terms shall
have the-meanings indicated below:

          "Beneficial owner" shall have the meaning defined in Rule 13d-3 under
     the Exchange Act.

          "Board" shall mean the Board of Directors of the Company.

          "Cause" for termination by the Company of the Executive's employment,
     after any Change in Control, shall mean (i) the willful and continued
     failure by the Executive to substantially perform the Executive's duties
     with the Company (other than any such failure resulting from the
     Executive's incapacity due to physical or mental illness or any such actual
     or anticipated failure after the issuance of a Notice of Termination for
     Good Reason by the Executive) after a written demand for substantial
     performance is delivered to the Executive by the Board, which demand
     specifically identifies the manner in which the Board believes that the
     Executive has not substantially performed the Executive's duties, or (ii)
     the willful engaging by the Executive in gross misconduct which is
     demonstrably and materially injurious to the Company or any of its
     subsidiaries, monetarily or otherwise. No act, or failure to act, on the
     Executive's part shall be deemed "willful" unless done, or omitted to be
     done, by the Executive not in good faith and without reasonable belief that
     the Executive's act, or failure to act, was in the best interest of the
     Company.

          A "Change in Control" shall be deemed to have occurred if the
     conditions set forth in any one of the following paragraphs shall have been
     satisfied:

               (a) Continuing Directors constitute two-thirds or less of the
          membership of the Board, whether as the result of a proxy contest or
          for any other reason or reasons; or

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               (b) Any Person is or becomes the Beneficial owner, directly or
          indirectly, of securities of the Company representing twenty-five
          percent or more of the combined voting power of the Company's then
          outstanding voting securities; or

               (c) There is a change in control of the Company of a nature that
          would be required to be reported on Form 8-K or item 6(e) of Schedule
          14A of Regulation 14A or any similar item, schedule or form under the
          Exchange Act, as in effect at the time of the change, whether or not
          the Company is then subject to such reporting requirement, including
          without limitation any merger or consolidation of the Company with any
          other corporation, other than (i) a merger or consolidation which
          would result in the voting securities of the Company outstanding
          immediately prior thereto continuing to represent (either by remaining
          outstanding or by being converted into voting securities of the
          surviving or parent entity) fifty-one percent or more of the combined
          voting power of the voting securities (entitled to vote generally for
          the election of directors) of the Company or such surviving or parent
          entity outstanding immediately after such merger or consolidation and
          which would result in those persons who are Continuing Directors
          immediately prior to such merger or consolidation constituting more
          than two-thirds of the membership of the Board or the board of such
          surviving or parent entity immediately after, or subsequently at any
          time as contemplated by or as a result of, such merger or
          consolidation or (ii) a merger or consolidation effected to implement
          a recapitalization of the company (or similar transaction) in which no
          Person acquired twenty-five percent or more of the combined voting
          power of the Company's then outstanding securities; or

               (d) the stockholders of the Company approve a plan of complete
          liquidation of the Company or an agreement for the sale or disposition
          by the Company of all or substantially all of the Company's assets (or
          any transaction having a similar effect).

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     "Code" shall mean the Internal Revenue Code of 1986, as amended from time
to time.

     "Company" shall mean Aspen Technology, Inc. and any successor to its
business and/or assets which assumes or agrees to perform this Agreement, by
operation of law or otherwise.

     "Compensation Committee" shall mean the Compensation and Nominating
Committee of the Board.

     "Continuing Director" shall mean any director (i) who has continuously been
a member of the Board since not later than the date of a Potential Change in
Control or (ii) who is a successor of a director described in clause (i), if
such successor (and any intervening successor) shall have been recommended or
elected to succeed a Continuing Director by a majority of the then Continuing
Directors.

     "Date of Termination" shall have the meaning stated in Section 5.2 hereof.

     "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended
from time to time.

     "Excise Tax" shall mean the tax imposed by Section 4999 of the Code.

     "Executive" shall mean the individual named in the first paragraph of this
Agreement.

     "Good Reason" for termination by the Executive of the Executive's
employment shall mean the occurrence (without the Executive's express written
consent) of any one of the following acts or failures to act by the Company
unless, in the case of any act or failure to act described in paragraph (a),
(e), (f) or (g) below, such act or failure to act is corrected prior to the Date
of Termination specified in the Notice of Termination given in respect thereof
or, in the case of paragraph (c) below, such act is not objected to in writing
by the Executive within four months after notification by the Company to the
Executive of the Company's intention to take the action contemplated by such
paragraph (c):

               (a) the assignment to the Executive of any duties inconsistent
          with the Executive's status as a senior executive officer of the
          Company or a meaningful

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          alteration, adverse to the Executive, in the nature or status of the
          Executive's responsibilities (other than reporting responsibilities)
          from those in effect immediately prior to the Change in Control;

               (b) a reduction by the Company in the Executive's annual base
          salary as in effect on the date hereof or as the same may be increased
          from time to time except for across-the-board salary reductions
          similarly affecting all senior executives of the Company and all
          senior executives of any Person in control of the Company;

               (c) the Company's requiring the Executive to be based anywhere
          other than the Boston Metropolitan Area (or, if different, the
          metropolitan area in which the Company's principal executive offices
          are located immediately prior to the Change in Control) except for
          required travel on the Company business to an extent substantially
          consistent with the Executive's present business travel obligations;

               (d) the failure by the Company, without the Executive's consent,
          to pay to the Executive any portion of the Executive's current
          compensation, or to pay to the Executive any portion of an installment
          of deferred compensation under any deferred compensation program of
          the Company, within fourteen days of the date such compensation is
          due;

               (e) the failure by the Company to continue in effect any
          compensation plan in which the Executive participates immediately
          prior to the Change in Control which is material to the Executive's
          total compensation, or the failure by the Company to continue the
          Executive's participation therein on a basis not materially less
          favorable, both in terms of the amount of benefits provided and the
          level of the Executive's participation relative to other participants,
          as existed at the time of the Change in Control;

               (f) the failure by the Company to continue to provide the
          Executive with benefits substantially similar to those enjoyed by the
          Executive under any of the Company' s pension, life insurance,
          medical, health and accident, or

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          disability plans in which the Executive was participating at the time
          of the Change in Control, the taking of any action by the Company
          which would directly or indirectly materially reduce any of such
          benefits or deprive the Executive of any material fringe benefit
          enjoyed by the Executive at the time of the Change in Control, or the
          failure by the Company to provide the Executive with the number of
          paid vacation days to which the Executive is entitled on the basis of
          years of service with the Company in accordance with the Company's
          normal vacation policy in effect at the time of the Change in Control;
          or

               (g) any purported termination of the Executive's employment which
          is not effected pursuant to a Notice of Termination satisfying the
          requirements of Section 5.1.

     "Notice of Termination" shall have the meaning stated in Section 5.1.

     "Person" shall have the meaning given in Section 3(a)(9) of the Exchange
Act, as modified and used in Sections 13(d) and 14(d). thereof; however, a
Person shall not include (i) the Company or any of its subsidiaries, (ii) a
trustee or other fiduciary holding securities under an employee benefit plan of
the Company or any of its subsidiaries, (iii) an underwriter temporarily holding
securities pursuant to a registered offering of such securities in accordance
with an agreement with the Company, or (iv) a corporation owned, directly or
indirectly, by the stockholders of the Company in substantially the same
proportions as their ownership of stock of the Company.

     "Potential Change in Control" shall be deemed to have occurred if the
conditions set forth in any one of the following paragraphs shall have been
satisfied:

               (a) the Company enters into an agreement, the consummation of
          which would result in the occurrence of a Change in Control;

               (b) the Company or any Person publicly announces an intention to
          take or to consider taking actions which, if consummated, would
          constitute a Change in Control;

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               (c) any Person becomes the Beneficial Owner, directly or
          indirectly, of securities of the Company representing fifteen percent
          or more of the combined voting power of the Company's then outstanding
          securities (entitled to vote generally for the election of directors);
          or

               (d) the Board adopts a resolution to the effect that, for
          purposes of this Agreement, a Potential Change in Control has
          occurred.

     "Severance Payments" shall mean those payments described in Section 3.2
hereof.

     "Total Payments" shall mean those payments described in Section 4 hereof.

     IN WITNESS WHEREOF, the Company and the Executive have executed and
delivered this Agreement on the date first written above.

                             ASPEN TECHNOLOGY, INC.

                             By:    /s/ Lawrence B. Evans
                                    --------------------------------

                                    /s/ Wayne Sim
                                    -------------------------------
                                        Wayne Sim<PAGE>
                                                                   Exhibit 10.50

September 30, 2002

Mr. David L. McQuillin
39 Julians Way
Sudbury, MA 01776

Dear David,

I am pleased to confirm you as President and Chief Executive Officer at Aspen
Technology, Inc. Your compensation will be as agreed to by the Compensation
Committee of the Board of Directors.

You will continue to receive the usual staff benefits which include paid
vacation, company paid holidays and company-paid life, AD&D and long -term
disability insurance.

Upon termination of employment for any reason other than disability, death,
resignation, or for Cause, AspenTech shall, in addition to any benefits due
under written plans, pay you only your base salary for 18 months after
termination of employment. "Cause" means material willful misconduct in the
discharge of your duties, your conviction of or entry of a plea of guilty or
nolo contender to any crime involving moral turpitude, or any material breach of
any term of your employment, including without limitation the Confidentiality
and Non-Competition Agreement.

Please confirm your acceptance of this employment letter by signing a copy and
returning it to me. AspenTech's Confidentiality and Non-Competition Agreement
will continue in force and effect.

Sincerely,

/s/ Lawrence B. Evans

Lawrence B. Evans
Chairman & Chief Executive Officer
Aspen Technology, Inc.

Accepted & Agreed To:

/s/ David L. McQuillin              9/30/02
--------------------------------    ----------------------------
David L. McQuillin                  Start Date

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