Senior VP Worldwide Sales Compensation Plan

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Exhibit 10.51


[AspenTechnology Letterhead/Logo]

June 24, 2003

Mr. Wayne Sim
Senior VP Worldwide Sales

Dear Wayne:

I am pleased to confirm that the Compensation Committee of the AspenTech Board of Directors has approved the following FY04 Executive Compensation Plan for you. This Plan is contingent upon shareholder approval of the Advent financing transaction, which is currently scheduled for August 2003.

Your total target compensation for FY04 will be $336,000, which is comprised of a base salary of $210,000 (this amount has been translated into US dollars based on an approximate exchange rate your actual salary is defined in Can ...