Senior VP Engineering Product Business Unit (pbu)

Exhibit 10.5 Aspen Technology, Inc. Ten Canal Park [phone] 617 949 1000 [world wide web] www.aspentech.com Cambridge MA 02141-2201 USA [fax] 617 949 1030 [e-mail] info@aspentech.com

[ASPENTECH LOGO]

June 24, 2003

Mr. Manolis Kotzabasakis
Senior VP Engineering Product Business Unit (PBU)

Dear Manolis:

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 $334,000, which is comprised of a base salary of $230,000 and a bonus target of 45%. The actual bonus you earn will be determined by your achievement of FY04 ...