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                                                                   EXHIBIT 10.18

                          REGISTRATION RIGHTS AGREEMENT

     THIS REGISTRATION RIGHTS AGREEMENT dated as of June 1, 2000 is entered into
between Aspen Technology, Inc., a Delaware corporation ("Aspen"), and David
Gamboa, Gordon Hartogensis and William Miller (collectively, the "Petrolsoft
Stockholders").

     A. Aspen, P-CAT Acquisition Corporation, Petrolsoft Corporation
("Petrolsoft") and the Petrolsoft Stockholders are entering into an Agreement
and Plan of Merger dated as of June 1, 2000 (the "Merger Agreement").

     B. Aspen and the Petrolsoft Stockholders desire to provide for certain
arrangements with respect to the registration of shares of common stock of Aspen
under the Securities Act of 1933.

     NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Agreement and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, and pursuant to
Section 5.2(h) of the Merger Agreement, the parties hereto agree as follows:

         1. Certain Definitions

         As used in this Agreement, the following terms shall have the following
     respective meanings:

         "Commission" means the Securities and Exchange Commission, or any other
     federal agency at the time administering the Securities Act.

         "Common Stock" means the common stock, $.10 par value per share, of
     Aspen.

         "Exchange Act" means the Securities Exchange Act of 1934, as amended,
     or any successor federal statute, and the rules and regulations of the
     Commission issued under such Act, as they each may, from time to time, be
     in effect.

         "Initiating Holders" means the Stockholders initiating a request for
     registration pursuant to Section 2.1(a).

         "Other Holders" shall have the meaning set forth in Section 2.1(c).

         "Prospectus" means the prospectus included in any Registration
     Statement, as amended or supplemented by an amendment or prospectus
     supplement, including post-effective amendments, and all material
     incorporated by reference or deemed to be incorporated by reference in such
     Prospectus.

         "Registration Statement" means a registration statement filed by Aspen
     with the Commission for a public offering and sale of securities of Aspen
     (other than a registration statement on Form S-8 or Form S-4, or their
     successors, or any other form for a similar limited purpose, or any
     registration statement covering only securities proposed to be issued in
     exchange for securities or assets of another corporation).

         "Registration Expenses" means the expenses described in Section 2.4.

         "Registrable Shares" means, with respect to a Stockholder, (a) the
     shares of Common Stock issued to such Stockholder pursuant to the Merger
     Agreement (including shares initially deposited in escrow pursuant to the
     terms of the Merger Agreement and subsequently delivered to such
     Stockholder), (b) any other securities issued by Aspen in exchange for any
     of such shares of Common Stock (but, with respect to any particular
     Registrable Share, only so long as it continues to be a
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     Registrable Share) and (c) any shares of Common Stock issued as a dividend
     or distribution on account of Registrable Shares or resulting from a
     subdivision of outstanding Registrable Shares into a greater number of
     shares (by reclassification, stock split or otherwise); provided that a
     security that was at one time a Registrable Share shall cease to be a
     Registrable Share when (i) it has been effectively registered and sold
     pursuant to a Registration Statement or (ii) it has been transferred and is
     no longer held of record by a Stockholder.

         "Securities Act" means the Securities Act of 1933, as amended, or any
     successor federal statute, and the rules and regulations of the Commission
     issued under such Act, as they each may, from time to time, be in effect.

         "Selling Stockholder" means any Stockholder owning Registrable Shares
     included in a Registration Statement.

         "Stockholders" means the Petrolsoft Stockholders and any other persons
     or entities constituting Stockholders pursuant to Section 3.

2.       Registration Rights

         2.1. Demand Registrations

                  (a) Request for Registration. On one or two occasions after
         September 15, 2000 (or after such earlier date as Aspen shall file its
         Annual Report on Form 10-K for the fiscal year ended June 30, 2000 with
         the Commission), a Stockholder or Stockholders holding a majority of
         the then-existing Registrable Shares ("Initiating Holders") may, by
         written request, require that Aspen register a portion of their
         Registrable Shares (subject to Section 2.1(b)) pursuant to a shelf
         Registration Statement on Form S-3 (a "Demand Registration"). Upon
         receipt of such a written request for a Demand Registration, Aspen
         shall promptly provide written notice of the proposed registration (the
         "Notice") to each remaining Stockholder. Each such remaining
         Stockholder may elect to have a portion of such Stockholder's
         Registrable Shares (subject to Section 2.1(b)) included in such Demand
         Registration by providing notice to Aspen within ten business days
         after delivery of Aspen's Notice. Thereupon, Aspen shall, as
         expeditiously as possible, use its best efforts to effect the Demand
         Registration of all Registrable Shares which Aspen has been requested
         to so register. For purposes of the first sentence of this Section
         2.1(a), Aspen shall use reasonable commercial efforts to file its
         Annual Report on Form 10-K for the fiscal year ended June 30, 2000 with
         the Commission as promptly as practicable after August 31, 2000.

                  (b) Share Limitation. In each Demand Registration, each
         Petrolsoft Stockholder, together with any other Stockholders holding
         Registrable Shares originally issued to such Petrolsoft Stockholder (a
         "Transferee"), may request registration of up to 220,092 Registrable
         Shares. If a Petrolsoft Stockholder and his Transferees have notified
         Aspen that they collectively wish to have more than 220,092 Registrable
         Shares registered pursuant to a Demand Registration, the 220,092
         Registrable Shares shall be allocated among the Petrolsoft Stockholder
         and his Transferees in proportion (as nearly as practicable) to the
         respective numbers of Registrable Shares held by them at the time Aspen
         delivered the Notice for such Demand Registration.

                  (c) Demand Registrations. Aspen shall not be required to
         effect more than two Demand Registrations pursuant to Section 2.1(a).
         In addition, Aspen shall not be required to effect the second Demand
         Registration prior to nine months after the termination of the initial
         Demand Registration. For purposes of this Section 2.1(c), a
         Registration Statement shall only be counted as a Demand Registration
         when it has been declared effective by the Commission (unless
         Initiating Holders withdraw their request for such registration (other
         than as a result of information concerning the

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         business or financial condition of Aspen which is made known to the
         Stockholders after the date on which such registration was requested)
         and elect not to pay the Registration Expenses therefor pursuant to
         Section 2.4).

                  (d) Deferral of Demand Registration. If at the time of a
         Demand Registration pursuant to Section 2.1(a),

                  (i)      Aspen is engaged or has plans to engage in a
                           registered public offering or any other activity
                           that, in the good faith determination of Aspen's
                           Board of Directors, would be adversely affected by
                           the requested Demand Registration, and

                  (ii)     Aspen's Board of Directors determines in good faith,
                           by appropriate resolutions, that, as a result of such
                           offering or other activity, (A) it would be
                           detrimental to Aspen (other than as relating solely
                           to the price of the Common Stock) to file the Demand
                           Registration at such time and (B) it is in the best
                           interests of Aspen to defer proceeding with the
                           Demand Registration at such time,

     then Aspen may direct that the filing of the Demand Registration be delayed
     for a period not to exceed (x) 120 days from the date of such request or
     (y) the period during which (in the good faith determination of Aspen's
     Board of Directors) filing the Demand Registration would be detrimental to
     Aspen, whichever occurs first. This right to delay filing a Demand
     Registration may not be exercised by Aspen more than once in any 12-month
     period.

                  (e) Suspension of Demand Registration. Aspen shall maintain
         the effectiveness of any Demand Registration for a period of up to 90
         days after the same has been first declared effective by the
         Commission, provided that Aspen may, by written notice to the Selling
         Stockholders, suspend or withdraw any such registration statement and
         require that the Selling Stockholders immediately cease the sale of
         shares of Aspen pursuant thereto if: (i) Aspen is engaged in any
         activity or transaction or preparations or negotiations for any
         activity or transaction that Aspen desires to keep confidential for
         business reasons, and Aspen determines in good faith that the public
         disclosure requirements imposed on Aspen pursuant to such registration
         statement would require disclosure of such activity or transaction;
         (ii) Aspen files a Registration Statement (other than a Registration
         Statement on Form S-4 or Form S-8, or any successor form) with the
         Commission for the purpose of registering under the Securities Act, any
         securities to be publicly offered and sold by Aspen; or (iii) Aspen
         fails to satisfy the requirements for use of Form S-3, as set forth in
         the general instructions to Form S-3. Upon receipt of such notice, each
         such Selling Stockholder shall immediately discontinue any sales of
         Registrable Shares pursuant to such Registration Statement until such
         Selling Stockholder has received copies of a supplemented or amended
         Prospectus or until such Selling Stockholder is advised in writing by
         Aspen that the then-current Prospectus may be used and has received
         copies of any additional or supplemental filings that are incorporated
         or deemed incorporated by reference in such Prospectus.

         2.2. Piggyback Registration

                  (a) Request for Inclusion and Best Efforts. If Aspen
         determines to file a Registration Statement (other than a Registration
         Statement filed pursuant to Section 2.1) (a "Piggyback Registration"),
         then Aspen shall promptly, prior to such filing, provide written notice
         to all Stockholders of its intention to do so, provided that no such
         notice is required to be given if no Registrable Shares are to be
         included therein as a result of a determination of the managing
         underwriter pursuant to Section 2.2(b). Upon the written request of a
         Stockholder or Stockholders given within 20 days after Aspen provides
         such notice (which request shall state the intended method of
         disposition of such Registrable Shares), Aspen shall use its best
         efforts to cause all Registrable

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         Shares which Aspen has been requested by such Stockholder or
         Stockholders to register to be registered under the Securities Act to
         the extent necessary to permit their sale or other disposition in
         accordance with the intended methods of distribution specified in the
         request of such Stockholder or Stockholders, provided that Aspen shall
         have the right to postpone or withdraw any registration effected
         pursuant to this Section 2.2 without obligation to any Stockholder.

                  (b) Underwriting. If the Piggyback Registration relates to an
         underwritten public offering, then Aspen shall so advise the
         Stockholders as a part of the written notice given pursuant to Section
         2.2(a). In such event, the right of any Stockholder to include its
         Registrable Shares in such registration pursuant to Section 2.2 shall
         be conditioned upon such Stockholder's participation in such
         underwriting on the terms set forth in this Agreement. All Stockholders
         proposing to distribute their securities through such underwriting
         shall enter into an underwriting agreement in customary form with the
         underwriter or underwriters selected for the underwriting by Aspen.
         Notwithstanding any other provision of this Section 2.2, if the
         managing underwriters determine that the inclusion of all shares
         requested to be registered would adversely affect the offering, then
         Aspen may limit the number of Registrable Shares to be included in the
         Piggyback Registration and shall so advise all holders of Registrable
         Shares requesting registration. The number of shares that are entitled
         to be included in the registration and underwriting shall be allocated
         in the following manner:

                  (i)      The securities of Aspen held by holders other than
                           Stockholders and other holders of securities of Aspen
                           who are entitled, by contract with Aspen, to have
                           their securities included in such registration (each
                           an "Other Holder") shall be excluded from such
                           Piggyback Registration to the extent deemed advisable
                           by the managing underwriters, and, if a further
                           limitation on the number of shares is required, then
                           the number of shares that may be included in such
                           Piggyback Registration shall be allocated pro rata
                           (on an as-converted basis) among all Stockholders and
                           Other Holders requesting registration in accordance
                           with the respective number of shares of Common Stock
                           held when Aspen provides notice as specified in
                           Section 2.2(a).

                  (ii)     If any Stockholder or Other Holder is entitled to
                           include more securities than such Stockholder or
                           Other Holder requested to be registered, then the
                           excess securities shall be allocated among other
                           requesting Stockholders and Other Holders pro rata in
                           the manner described in the preceding clause (i).

     If any holder of Registrable Shares or any officer, director or Other
     Holder disapproves of the terms of any such underwriting, such person may
     elect to withdraw therefrom by written notice to Aspen, and any Registrable
     Shares or other securities excluded or withdrawn from such underwriting
     shall be withdrawn from such registration.

                  (c) Rule 144(k). Notwithstanding the foregoing, Aspen shall
         not be required, pursuant to this Section 2.2, to include any
         Registrable Shares in a Registration Statement if such Registrable
         Shares can then be sold pursuant to Rule 144(k) under the Securities
         Act.

         2.3. Registration Procedures

                  (a) General. If and whenever Aspen is required by the
         provisions of this Agreement to use its best efforts to effect the
         registration of any Registrable Shares under the Securities Act, Aspen
         shall:

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                     (i)    prepare and file with the Commission a Registration
                            Statement on an appropriate form as soon as
                            practicable, and cause such Registration Statement
                            to be declared effective by the Commission at the
                            earliest practicable date;

                     (ii)   as expeditiously as possible prepare and file with
                            the Commission any amendments and supplements to the
                            Registration Statement and the Prospectus included
                            in the Registration Statement as may be necessary to
                            comply with the provisions of the Securities Act
                            (including the anti-fraud provisions thereof) and to
                            keep the Registration Statement effective for 90
                            days from the effective date or such lesser period
                            until all such Registrable Shares are sold;

                     (iii)  as expeditiously as possible furnish to each Selling
                            Stockholder such reasonable numbers of copies of the
                            Prospectus, including any preliminary Prospectus, in
                            conformity with the requirements of the Securities
                            Act, and such other documents as such Selling
                            Stockholder may reasonably request in order to
                            facilitate the public sale or other disposition of
                            the Registrable Shares owned by such Selling
                            Stockholder;

                     (iv)   as expeditiously as possible use its best efforts to
                            register or qualify the Registrable Shares covered
                            by the Registration Statement under the securities
                            or Blue Sky laws of such states as the Selling
                            Stockholders shall reasonably request, and do any
                            and all other acts and things that may be necessary
                            or desirable to enable the Selling Stockholders to
                            consummate the public sale or other disposition in
                            such states of the Registrable Shares owned by the
                            Selling Stockholder, provided that Aspen shall not
                            be required in connection with this paragraph (iv)
                            to qualify as a foreign corporation or execute a
                            general consent to service of process in any
                            jurisdiction;

                     (v)    as expeditiously as possible, cause all such
                            Registrable Shares to be listed on each securities
                            exchange or automated or inter-dealer quotation
                            system on which similar securities issued by Aspen
                            are then listed;

                     (vi)   promptly provide a transfer agent and registrar for
                            all such Registrable Shares no later than the
                            effective date of such registration statement;

                     (vii)  promptly make available for inspection by the
                            Selling Stockholders, any managing underwriter
                            participating in any disposition pursuant to such
                            Registration Statement, and any attorney or
                            accountant or other agent retained by any such
                            underwriter or selected by the Selling Stockholders,
                            all financial and other records, pertinent corporate
                            documents and properties of Aspen and cause Aspen's
                            officers, directors, employees and independent
                            accountants to supply all information reasonably
                            requested by any such seller, underwriter, attorney,
                            accountant or agent in connection with such
                            Registration Statement;

                     (viii) notify each Selling Stockholder at any time when a
                            Prospectus relating to the Registration Statement is
                            required to be delivered under the Securities Act,
                            of the happening of any event as a result of which
                            the Prospectus included in such Registration
                            Statement, as then in effect, includes an untrue
                            statement of a material fact or omits to state a
                            material fact required to be stated therein or
                            necessary to make the statement therein not
                            misleading or incomplete in light of the
                            circumstances then existing, and promptly amend the
                            Registration Statement

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                            and/or related Prospectus to correct such untrue
                            statement or to include the omitted information;

                     (ix)   as expeditiously as possible, notify each Selling
                            Stockholder, promptly after it shall receive notice
                            thereof, of the time when such Registration
                            Statement has become effective or a supplement to
                            any Prospectus forming a part of such Registration
                            Statement has been filed; and

                     (x)    as expeditiously as possible following the
                            effectiveness of such Registration Statement, notify
                            each seller of such Registrable Shares of any
                            request by the Commission for the amending or
                            supplementing of such Registration Statement or
                            Prospectus.

                (b) Prospective Amendments. If Aspen has delivered a Prospectus
     to the Selling Stockholders and after having done so the Prospectus is
     amended to comply with the requirements of the Securities Act, Aspen shall
     promptly notify the Selling Stockholders and, if requested, the Selling
     Stockholders shall immediately cease making offers of Registrable Shares
     and return all Prospectuses to Aspen. Aspen shall promptly provide the
     Selling Stockholders with revised Prospectuses and, following receipt of
     the revised Prospectuses, the Selling Stockholders shall be free to resume
     making offers of the Registrable Shares.

         2.4. Allocation of Expenses. Aspen will pay all Registration Expenses
for all registrations under this Agreement; provided, however, that if a Demand
Registration under Section 2.1 is withdrawn at the request of the Initiating
Holders (other than as a result of information concerning the business or
financial condition of Aspen which is made known to the Stockholders after the
date on which such registration was requested) and if the Initiating Holders
elect not to have such registration counted as a registration requested under
Section 2.1(a), then the Initiating Holders shall pay the Registration Expenses
of such registration pro rata in accordance with the number of their Registrable
Shares included in the Demand Registration. For purposes of this Section, the
term "Registration Expenses" shall mean all reasonable expenses incurred in
effecting any registration pursuant to this Agreement, including all
registration and filing fees, exchange listing fees, printing expenses, fees and
expenses of counsel for Aspen and the fees and expenses of one counsel selected
by the Selling Stockholders to represent the Selling Stockholders, state Blue
Sky fees and expenses, and the expense of any regular or special audits incident
to or required by any such registration, but excluding underwriting discounts,
selling commissions and the fees and expenses of Selling Stockholders' own
counsel (other than the counsel selected to represent all Selling Stockholders).

         2.5.   Indemnification and Contribution.

                (a) Indemnification by Aspen. In the event of any registration
     of any of the Registrable Shares under the Securities Act pursuant to this
     Agreement, Aspen will indemnify and hold harmless each Selling Stockholder,
     each underwriter of such Registrable Shares, and each other person, if any,
     who controls such Selling Stockholder or underwriter within the meaning of
     the Securities Act or the Exchange Act, with respect to each registration,
     qualification or compliance effected pursuant to this Section 2, against
     any losses, claims, damages, liabilities (or actions, proceedings or
     settlements in respect thereof), joint or several, to which such Selling
     Stockholder, underwriter or controlling person may become subject under the
     Securities Act, the Exchange Act, state securities or Blue Sky laws or
     otherwise, insofar as such losses, claims, damages or liabilities (or
     actions, proceedings or settlements in respect thereof) arise out of or are
     based upon any untrue statement or alleged untrue statement of any material
     fact contained in any Registration Statement under which such Registrable
     Shares were registered under the Securities Act, any preliminary prospectus
     or final prospectus contained in the Registration Statement, or any
     amendment or supplement to such Registration Statement, incident to

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     any such registration, qualification or compliance, or arise out of or are
     based upon the omission or alleged omission to state a material fact
     required to be stated therein or necessary to make the statements therein
     not misleading; or any violation by Aspen of the Securities Act or Exchange
     Act or any rule or regulation thereunder applicable to Aspen and relating
     to action or inaction required by Aspen in connection with any such
     registration, qualification or compliance; and Aspen will reimburse such
     Selling Stockholder, underwriter and each such controlling person for any
     legal or any other expenses reasonably incurred by such Selling
     Stockholder, underwriter or controlling person in connection with
     investigating or defending any such loss, claim, damage, liability or
     action; provided, however, that Aspen will not be liable in any such case
     to the extent that any such loss, claim, damage or liability arises out of
     or is based upon any untrue statement or omission made in such Registration
     Statement, preliminary prospectus or prospectus, or any such amendment or
     supplement, in reliance upon and in conformity with written information
     furnished to Aspen, by or on behalf of such Selling Stockholder,
     underwriter or controlling person and stated to be specifically for use in
     the preparation thereof.

                (b) Indemnification by Selling Stockholders. In the event of any
     registration of any of the Registrable Shares under the Securities Act
     pursuant to this Agreement, each Selling Stockholder, severally and not
     jointly, will indemnify and hold harmless Aspen, each of its directors and
     officers and each underwriter (if any) and each person, if any, who
     controls Aspen or any such underwriter within the meaning of the Securities
     Act or the Exchange Act, against any losses, claims, damages or
     liabilities, joint or several, to which Aspen, such directors and officers,
     underwriter or controlling person may become subject under the Securities
     Act, Exchange Act, state securities or Blue Sky laws or otherwise, insofar
     as such losses, claims, damages or liabilities (or actions in respect
     thereof) arise out of or are based upon any untrue statement or alleged
     untrue statement of a material fact contained in any Registration Statement
     under which such Registrable Shares were registered under the Securities
     Act, any preliminary prospectus or final prospectus contained in the
     Registration Statement, or any amendment or supplement to the Registration
     Statement, or arise out of or are based upon any omission or alleged
     omission to state a material fact required to be stated therein or
     necessary to make the statements therein not misleading, if the statement
     or omission was made in reliance upon and in conformity with information
     relating to such Selling Stockholder furnished in writing to Aspen by or on
     behalf of such Selling Stockholder specifically for use in connection with
     the preparation of such Registration Statement, prospectus, amendment or
     supplement; provided, however, that the obligations of a Selling
     Stockholder hereunder shall be limited to an amount equal to the net
     proceeds to such Selling Stockholder of Registrable Shares sold in
     connection with such registration.

                (c) Indemnification Procedures. Each party entitled to
     indemnification under this Section (the "Indemnified Party") shall give
     notice to the party required to provide indemnification (the "Indemnifying
     Party") promptly after such Indemnified Party has actual knowledge of any
     claim as to which indemnity may be sought, and shall permit the
     Indemnifying Party to assume the defense of any such claim or any
     litigation resulting therefrom; provided, that counsel for the Indemnifying
     Party, who shall conduct the defense of such claim or litigation, shall be
     approved by the Indemnified Party (whose approval shall not be unreasonably
     withheld); and, provided, further, that the failure of any Indemnified
     Party to give notice as provided herein shall not relieve the Indemnifying
     Party of its obligations under this Section except to the extent that the
     Indemnifying Party is adversely affected by such failure. The Indemnified
     Party may participate in such defense at such party's expense; provided
     that the Indemnifying Party shall pay such expense if representation of
     such Indemnified Party by the counsel retained by the Indemnifying Party
     would be inappropriate due to actual or potential differing interests
     between the Indemnified Party and any other party represented by such
     counsel in such proceeding; provided, further, that in no event shall the
     Indemnifying Party be required to pay the expenses of more than one law
     firm per jurisdiction as counsel for the Indemnified

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     Party. The Indemnifying Party also shall be responsible for the expenses of
     such defense if the Indemnifying Party does not elect to assume such
     defense. No Indemnifying Party, in the defense of any such claim or
     litigation shall, except with the consent of each Indemnified Party,
     consent to entry of any judgment or enter into any settlement which does
     not include as an unconditional term thereof the giving by the claimant or
     plaintiff to such Indemnified Party of a release from all liability in
     respect of such claim or litigation, and no Indemnified Party shall consent
     to entry of any judgment or settle such claim or litigation without the
     prior written consent of the Indemnifying Party, which consent shall not be
     unreasonably withheld.

                (d) Contribution. In order to provide for just and equitable
     contribution in circumstances in which the indemnification provided for in
     this Section 2.5 is due in accordance with its terms but for any reason is
     held to be unavailable to an Indemnified Party in respect to any losses,
     claims, damages and liabilities referred to herein, then the Indemnifying
     Party shall, in lieu of indemnifying such Indemnified Party, contribute to
     the amount paid or payable by such Indemnified Party as a result of such
     losses, claims, damages or liabilities to which such party may be subject
     in such proportion as is appropriate to reflect the relative fault of Aspen
     on the one hand and the Selling Stockholders on the other in connection
     with the statements or omissions which resulted in such losses, claims,
     damages or liabilities, as well as any other relevant equitable
     considerations. The relative fault of Aspen and the Selling Stockholders
     shall be determined by reference to, among other things, whether the untrue
     or alleged untrue statement of material fact related to information
     supplied (or required to be supplied) by Aspen or the Selling Stockholders
     and the parties' relative intent, knowledge, access to information and
     opportunity to correct or prevent such statement or omission. Aspen and the
     Selling Stockholders agree that it would not be just and equitable if
     contribution pursuant to this Section 2.5 were determined by pro rata
     allocation or by any other method of allocation which does not take account
     of the equitable considerations referred to above. Notwithstanding the
     provisions of this paragraph of Section 2.5, (a) in no case shall any one
     Selling Stockholder be liable or responsible for any amount in excess of
     the net proceeds received by such Selling Stockholder from the offering of
     Registrable Shares and (b) Aspen shall be liable and responsible for any
     amount in excess of such proceeds; provided that no person guilty of
     fraudulent misrepresentation (within the meaning of Section 11(f) of the
     Securities Act) shall be entitled to contribution from any person who was
     not guilty of such fraudulent misrepresentation. Any party entitled to
     contribution will, promptly after receipt of notice of commencement of any
     action, suit or proceeding against such party in respect of which a claim
     for contribution may be made against another party or parties under this
     Section, notify such party or parties from whom contribution may be sought,
     but the omission so to notify such party or parties from whom contribution
     may be sought shall not relieve such party from any other obligation it or
     they may have thereunder or otherwise under this Section. No party shall be
     liable for contribution with respect to any action, suit, proceeding or
     claim settled without its prior written consent, which consent shall not be
     unreasonably withheld.

         2.6. Other Matters with Respect to Underwritten Offerings. In the event
that Registrable Shares are sold pursuant to a Registration Statement in an
underwritten offering pursuant to Section 2.1, Aspen agrees to: (a) enter into
an underwriting agreement containing customary representations and warranties
with respect to the business and operations of Aspen and customary covenants and
agreements to be performed by Aspen, including without limitation customary
provisions with respect to indemnification by Aspen of the underwriters of such
offering; (b) use its best efforts to cause its legal counsel to render
customary opinions to the underwriters with respect to the Registration
Statement; and (c) use its best efforts to cause its independent public
accounting firm to issue a customary "cold comfort letter" to the underwriters
with respect to the Registration Statement.

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         2.7. Information by Holder. Each Stockholder included in any
registration shall furnish to Aspen such information regarding such Stockholder
and the distribution proposed by such Stockholder as Aspen may reasonably
request in writing and as shall be required in connection with any registration,
qualification or compliance referred to in this Agreement.

         2.8. "Stand-Off" Agreement; Confidentiality of Notices. Each
Stockholder, if requested by Aspen and the managing underwriters of an
underwritten public offering by Aspen of Common Stock, shall not sell or
otherwise transfer or dispose of any Registrable Shares or other securities of
Aspen held by such Stockholder for a period of 180 days following the effective
date of a Registration Statement. Aspen may impose stop-transfer instructions
with respect to the Registrable Shares or other securities subject to the
foregoing restriction until the end of such 180-day period. Any Stockholder
receiving any written notice from Aspen regarding Aspen's plans to file a
Registration Statement shall treat such notice confidentially and shall not
disclose such information to any person other than as necessary to exercise his
rights under this Agreement.

         2.9.   Rule 144 Requirements.  Aspen agrees to:

                (a) make and keep current public information about Aspen
                    available, as those terms are understood and defined in Rule
                    144;

                (b) use its best efforts to prepare and file with the Commission
                    in a timely manner all reports and other documents required
                    of Aspen under the Securities Act and the Exchange Act (at
                    any time after it has become subject to such reporting
                    requirements); and

                (c) promptly furnish to any holder of Registrable Shares upon
                    request (A) a written statement by Aspen as to its
                    compliance with the reporting requirements of Rule 144 and
                    of the Securities Act and the Exchange Act (at any time
                    after it has become subject to such reporting requirements),
                    (B) a copy of the most recent annual or quarterly report of
                    Aspen, and (C) such other reports and documents of Aspen as
                    such holder may reasonably request to avail itself of any
                    similar rule or regulation of the Commission allowing it to
                    sell any such securities without registration.

         2.10. Sections 2.1 and 2.2 Termination. All of Aspen's obligations to
register Registrable Shares under Sections 2.1 and 2.2 shall terminate on June
1, 2002. In addition, all of the rights granted to a Stockholder under Sections
2.1 and 2.2 shall terminate as of any earlier date on which all of the
Registrable Shares of such Stockholder may be sold within a three-month period
pursuant to Rule 144.

     3.  Transfers of Rights

         This Agreement, and the rights and obligations of each Stockholder
hereunder, may be assigned by such Stockholder to any person or entity to which
at least 25,000 Shares are transferred by such Stockholder, and such transferee
shall be deemed a "Stockholder" for purposes of this Agreement; provided that
the transferee provides written notice of such assignment to Aspen and agrees in
writing to be bound hereby.

     4.  General

         4.1. No Third-Party Beneficiaries. This Agreement shall not confer any
rights or remedies upon any person other than the parties hereto and their
respective successors and permitted assigns pursuant to Section 3.

                                      -9-
<PAGE>   10
         4.2. Entire Agreement. This Agreement constitutes the entire agreement
among the parties hereto with respect to the subject matter hereof and
supersedes any prior understandings, agreements or representations by or among
the parties hereto, written or oral, with respect to the subject matter hereof.

         4.3. Succession and Assignment. This Agreement shall be binding upon
and inure to the benefit of the parties named herein and their respective
successors and permitted assigns pursuant to Section 3.

         4.4. Counterparts and Facsimile Signature. This Agreement may be
executed in two or more counterparts, each of which shall be deemed an original
but all of which together shall constitute one and the same instrument. This
Agreement may be executed by facsimile signature.

         4.5. Headings. The section headings contained in this Agreement are
inserted for convenience only and shall not affect in any way the meaning or
interpretation of this Agreement.

         4.6. Notices. All notices, instructions, demands, claims, requests and
other communications given hereunder or in connection herewith shall be in
writing. Any such communication shall be sent either (a) by registered or
certified mail, return receipt requested, postage prepaid, or (b) via a
reputable nationwide overnight courier service, in each case to the address set
forth below. Any such communication shall be deemed to have been delivered two
business days after it is sent by registered or certified mail, return receipt
requested, postage prepaid, or one business day after it is sent via a reputable
nationwide overnight courier service.

        To Aspen:                           Aspen Technology, Inc.
                                            Ten Canal Park
                                            Cambridge, Massachusetts  02141
                                            Facsimile:  617.577.0722
                                            Attention:  Chief Executive Officer

             With a copy to:                Hale and Dorr LLP
                                            60 State Street
                                            Boston, Massachusetts  02109
                                            Facsimile:  617.526.5000
                                            Attention: Mark L. Johnson

        To any Petrolsoft Stockholders,
             to his attention in care of:   Petrolsoft Corporation
                                            12730 High Bluff Drive
                                            San Diego, California  92130
                                            Facsimile:  858.259.9725
                                            Attention:  Chief Executive Officer

             With a copy to:                Brobeck, Phleger & Harrison LLP
                                            12390 El Camino Real
                                            San Diego, California 92130
                                            Facsimile:  858.720.2550
                                            Attention:  Eddie Rodriguez

        To any other Stockholder:           At such Stockholder's address of
                                            record in the stock transfer records
                                            of Aspen

                                      -10-
<PAGE>   11
Any party hereto may give any notice, instruction, demand, claim, request or
other communication hereunder using any other means (including personal
delivery, expedited courier, messenger service, telecopy, telex, ordinary mail
or electronic mail), but no such communication shall be deemed to have been duly
given unless and until it actually is received by the party for which it is
intended. Any party hereto may change the address to which notices,
instructions, demands, claims, requests and other communications hereunder are
to be delivered by giving the other parties notice in the manner set forth in
this Section 4.6.

         4.7. Governing Law. This Agreement shall be governed by and construed
in accordance with the internal laws of the Commonwealth of Massachusetts
without giving effect to any choice or conflict of law provision or rule
(whether of the Commonwealth of Massachusetts or any other jurisdiction) that
would cause the application of laws of any jurisdictions other than those of the
Commonwealth of Massachusetts.

         4.8. Amendments and Waivers. Any term of this Agreement may be amended
or terminated and the observance of any term of this Agreement may be waived
(either generally or in a particular instance and either retroactively or
prospectively), with the written consent of Aspen and the holders of at least a
majority of the Registrable Shares held by all of the Stockholders. Any such
amendment, termination or waiver effected in accordance with this Section 4.8
shall be binding on all parties hereto, even if they do not execute such
consent. No waivers of or exceptions to any term, condition or provision of this
Agreement, in any one or more instances, shall be deemed to be, or construed as,
a further or continuing waiver of any such term, condition or provision.

         4.9. Severability. Any term or provision of this Agreement that is
invalid or unenforceable in any situation in any jurisdiction shall not affect
the validity or enforceability of the remaining terms and provisions hereof or
the validity or enforceability of the offending term or provision in any other
situation or in any other jurisdiction. If the final judgment of a court of
competent jurisdiction declares that any term or provision hereof is invalid or
unenforceable, the parties hereto agree that the court making the determination
of invalidity or unenforceability shall have the power to limit the term or
provision, to delete specific words or phrases, or to replace any invalid or
unenforceable term or provision with a term or provision that is valid and
enforceable and that comes closest to expressing the intention of the invalid or
unenforceable term or provision, and this Agreement shall be enforceable as so
modified.

         4.10.  Construction.

                (a) The language used in this Agreement shall be deemed to be
     the language chosen by the parties hereto to express their mutual intent,
     and no rule of strict construction shall be applied against any party
     hereto.

                (b) The term "including" as used herein shall not be construed
     so as to exclude any other thing not referred to or described.

                (c) References herein to "Sections" shall be deemed to be to
     sections of this Agreement, unless otherwise specified.

                                      -11-
<PAGE>   12
     IN WITNESS WHEREOF, the parties have executed this Agreement as of June 1,
2000.

                                   ASPEN TECHNOLOGY, INC.

                                   By:      /s/ David Mushin
                                        Title:  Executive Vice President

                                   STOCKHOLDERS:

                                            /s/ David Gamboa
                                   ---------------------------------------------
                                   DAVID GAMBOA, as Trustee of the Gamboa Family
                                   Trust dated August 5, 1996

                                            /s/ Gordon Hartogensis
                                   ---------------------------------------------
                                   GORDON HARTOGENSIS

                                            /s/ William Miller
                                   ---------------------------------------------
                                   WILLIAM MILLER

                                      -12-<PAGE>   1
                                                                   EXHIBIT 10.19

                          REGISTRATION RIGHTS AGREEMENT

     THIS REGISTRATION RIGHTS AGREEMENT dated as of August 29, 2000 is entered
into between Aspen Technology, Inc., a Delaware corporation ("Aspen"), and
Eunice E. Blecker and Herbert G. Blecker (together, the "Stockholders").

     A. Aspen, Iris Acquisition Corporation, ICARUS Corporation, ICARUS Services
Limited and the Stockholders are parties to an Agreement and Plan of Acquisition
dated as of the date hereof (the "Acquisition Agreement").

     B. Aspen and the Stockholders desire to provide for certain arrangements
with respect to the registration under the Securities Act of 1933 of shares of
common stock of Aspen issued to the Stockholders pursuant to the Acquisition
Agreement.

     NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Agreement and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, and pursuant to
Section 7.2(b) of the Acquisition Agreement, the parties hereto agree as
follows:

1.   Certain Definitions

         As used in this Agreement, the following terms shall have the following
     respective meanings:

         "Business Day" means any day that the Securities and Exchange
     Commission is open and conducting business.

         "Commission" means the Securities and Exchange Commission, or any other
     federal agency at the time administering the Securities Act.

         "Common Stock" means the common stock, $.10 par value per share, of
     Aspen.

         "Exchange Act" means the Securities Exchange Act of 1934, as amended,
     or any successor federal statute, and the rules and regulations of the
     Commission issued under such Act, as they each may, from time to time, be
     in effect.

         "Initiating Holders" means the Stockholders initiating a request for
     registration pursuant to Section 2.1(a).

         "Other Holders" shall have the meaning set forth in Section 2.2(b)(i).

         "Prospectus" means the prospectus included in any Registration
     Statement, as amended or supplemented by an amendment or prospectus
     supplement, including post-effective amendments, and all material
     incorporated by reference or deemed to be incorporated by reference in such
     Prospectus.

         "Registration Statement" means a registration statement filed by Aspen
     with the Commission for a public offering and sale of securities of Aspen
     (other than a registration statement on Form S-8 or Form S-4, or their
     successors, or any other form for a similar limited purpose, or any
     registration statement covering only securities proposed to be issued in
     exchange for securities or assets of another corporation).

         "Registration Expenses" means the expenses described in Section 2.4.
<PAGE>   2
         "Registrable Shares" means, with respect to a Stockholder, (a) the
     shares of Common Stock issued to such Stockholder pursuant to the
     Acquisition Agreement (including shares initially deposited in escrow
     pursuant to the terms of the Acquisition Agreement and subsequently
     delivered to such Stockholder), (b) any other securities issued by Aspen in
     exchange for any of such shares of Common Stock (but, with respect to any
     particular Registrable Share, only so long as it continues to be a
     Registrable Share) and (c) any shares of Common Stock issued as a dividend
     or distribution on account of Registrable Shares or resulting from a
     subdivision of outstanding Registrable Shares into a greater number of
     shares (by reclassification, stock split or otherwise); provided that a
     security that was at one time a Registrable Share shall cease to be a
     Registrable Share when (i) it has been effectively registered and sold
     pursuant to a Registration Statement or (ii) it has been transferred and is
     no longer held of record by a Stockholder.

         "Securities Act" means the Securities Act of 1933, as amended, or any
     successor federal statute, and the rules and regulations of the Commission
     issued under such Act, as they each may, from time to time, be in effect.

         "Selling Stockholder" means any Stockholder owning Registrable Shares
     included in a Registration Statement.

         "Stockholders" means the Stockholders and any other persons or entities
     constituting Stockholders pursuant to Section 3.

2.   Registration Rights

         2.1.   Demand Registrations

                (a) Request for Registration. On up to two occasions after
     October 31, 2000, a Stockholder or Stockholders holding a majority of the
     then-existing Registrable Shares ("Initiating Holders") may, by written
     request but subject to Section 2.1(b), require that Aspen register all or a
     portion of their Registrable Shares pursuant to a shelf Registration
     Statement on Form S-3 (a "Demand Registration"). Upon receipt of such a
     written request for a Demand Registration, Aspen shall promptly provide
     written notice of the proposed registration (the "Notice") to each
     remaining Stockholder. Each such remaining Stockholder may, subject to
     Section 2.1(b), elect to have all or a portion of such Stockholder's
     Registrable Shares included in such Demand Registration by providing notice
     to Aspen within five Business Days after delivery of the Notice. Thereupon,
     Aspen shall comply with the registration procedures of Section 2.3 with
     respect to the Demand Registration of all Registrable Shares that Aspen has
     been requested to so register.

                (b) Share Limitation. In each Demand Registration, each
     Stockholder may request registration of all or a portion of the Registrable
     Shares held by such Stockholder. If the Stockholders have notified Aspen
     that they collectively wish to have more than 124,206 Registrable Shares
     registered pursuant to a Demand Registration, the 124,206 Registrable
     Shares shall be allocated among the Stockholders in proportion (as nearly
     as practicable) to the respective numbers of Registrable Shares held by
     them at the time Aspen delivered the Notice for such Demand Registration.

                (c) Demand Registrations. Aspen shall not be required to effect
     more than two Demand Registrations pursuant to Section 2.1(a). In addition,
     Aspen shall not be required to effect the second Demand Registration prior
     to three months after the termination of the initial Demand Registration.
     For purposes of this Section 2.1(c), a Registration Statement shall only be
     counted as a Demand Registration when it has been declared effective by the
     Commission (unless Initiating Holders withdraw their request for such
     registration (other than as a result of information concerning the

                                       2
<PAGE>   3
     business or financial condition of Aspen that is made known to the
     Stockholders after the date on which such registration was requested) and
     elect not to pay the Registration Expenses therefor pursuant to Section
     2.4).

         (d) Deferral of Demand Registration. If at the time of a Demand
     Registration pursuant to Section 2.1(a),

                     (i)  Aspen is engaged or has plans to engage in a
                          registered public offering or any other activity that,
                          in the good faith determination of Aspen's Board of
                          Directors, would be adversely affected by the
                          requested Demand Registration, and

                     (ii) Aspen's Board of Directors determines in good faith,
                          by appropriate resolutions, that, as a result of such
                          offering or other activity, (A) it would be
                          detrimental to Aspen (other than as relating solely to
                          the price of the Common Stock) to file the Demand
                          Registration at such time and (B) it is in the best
                          interests of Aspen to defer proceeding with the Demand
                          Registration at such time,

     then Aspen may direct that the filing of the Demand Registration be delayed
     for a period not to exceed (x) 120 days from the date of such request or
     (y) the period during which (in the good faith determination of Aspen's
     Board of Directors) filing the Demand Registration would be detrimental to
     Aspen, whichever occurs first. This right to delay filing a Demand
     Registration may not be exercised by Aspen more than once in any 12-month
     period.

                (e) Suspension of Demand Registration. Aspen shall maintain the
     effectiveness of any Demand Registration for a period of up to 90 days
     after the same has been first declared effective by the Commission,
     provided that Aspen may, by written notice to the Selling Stockholders,
     suspend or withdraw any such registration statement and require that the
     Selling Stockholders immediately cease the sale of shares of Aspen pursuant
     thereto if:

                     (i)    Aspen is engaged in any activity or transaction or
                            preparations or negotiations for any activity or
                            transaction that Aspen desires to keep confidential
                            for business reasons, and Aspen's Board of Directors
                            determines in good faith, by appropriate
                            resolutions, that the public disclosure requirements
                            imposed on Aspen pursuant to such registration
                            statement would require disclosure of such activity
                            or transaction;

                     (ii)   Aspen files a Registration Statement (other than a
                            Registration Statement on Form S-4 or Form S-8, or
                            any successor form) with the Commission for the
                            purpose of registering under the Securities Act, any
                            securities to be publicly offered and sold by Aspen;
                            or

                     (iii)  Aspen fails to satisfy the requirements for use of
                            Form S-3, as set forth in the general instructions
                            to Form S-3.

     Upon receipt of such notice, each such Selling Stockholder shall
     immediately discontinue any sales of Registrable Shares pursuant to such
     Registration Statement until such Selling Stockholder has received copies
     of a supplemented or amended Prospectus or until such Selling Stockholder
     is advised in writing by Aspen that the then-current Prospectus may be used
     and has received copies of any additional or supplemental filings that are
     incorporated or deemed incorporated by reference in such Prospectus.

                                       3
<PAGE>   4
         2.2.   Piggyback Registration

                (a) Request for Inclusion and Best Efforts. If Aspen determines
     to file a Registration Statement for an underwritten public offering (other
     than a Registration Statement filed pursuant to Section 2.1) (a "Piggyback
     Registration"), then Aspen shall promptly, prior to such filing, provide
     written notice to all Stockholders of its intention to do so, provided that
     no such notice is required to be given if no Registrable Shares are to be
     included therein as a result of a determination of the managing underwriter
     pursuant to Section 2.2(b). Upon the written request of a Stockholder or
     Stockholders given within 20 days after Aspen provides such notice (which
     request shall state the intended method of disposition of such Registrable
     Shares), Aspen shall use its best efforts to cause all Registrable Shares
     that Aspen has been requested by such Stockholder or Stockholders to
     register to be registered under the Securities Act to the extent necessary
     to permit their sale or other disposition in accordance with the intended
     methods of distribution specified in the request of such Stockholder or
     Stockholders, provided that Aspen shall have the right to postpone or
     withdraw any registration effected pursuant to this Section 2.2 without
     obligation to any Stockholder.

                (b) Underwriting. The right of any Stockholder to include its
     Registrable Shares in such registration pursuant to Section 2.2 shall be
     conditioned upon such Stockholder's participation in the contemplated
     underwritten public offering on the terms set forth in this Agreement. All
     Stockholders proposing to distribute their securities through such
     underwriting shall enter into an underwriting agreement in customary form
     with the underwriter or underwriters selected for the underwriting by
     Aspen. Notwithstanding any other provision of this Section 2.2, if the
     managing underwriters determine that the inclusion of all shares requested
     to be registered would adversely affect the offering, then Aspen may limit
     the number of Registrable Shares to be included in the Piggyback
     Registration and shall so advise all holders of Registrable Shares
     requesting registration. The number of shares that are entitled to be
     included in the registration and underwriting shall be allocated in the
     following manner:

                     (i)    The securities of Aspen held by holders other than
                            Stockholders and other holders of securities of
                            Aspen who are entitled, by contract with Aspen, to
                            have their securities included in such registration
                            (each an "Other Holder") shall be excluded from such
                            Piggyback Registration to the extent deemed
                            advisable by the managing underwriters, and, if a
                            further limitation on the number of shares is
                            required, then the number of shares that may be
                            included in such Piggyback Registration shall be
                            allocated pro rata (on an as-converted basis) among
                            all Stockholders and Other Holders requesting
                            registration in accordance with the respective
                            number of shares of Common Stock held when Aspen
                            provides notice as specified in Section 2.2(a).

                     (ii)   If any Stockholder or Other Holder is entitled to
                            include more securities than such Stockholder or
                            Other Holder requested to be registered, then the
                            excess securities shall be allocated among other
                            requesting Stockholders and Other Holders pro rata
                            in the manner described in the preceding clause (i).

     If any holder of Registrable Shares or Other Holder disapproves of the
     terms of any such underwriting, such person may elect to withdraw therefrom
     by written notice to Aspen, and any Registrable Shares or other securities
     excluded or withdrawn from such underwriting shall be withdrawn from such
     registration.

                                       4
<PAGE>   5
         2.3.   Registration Procedures

                (a) General. If and whenever Aspen is required by the provisions
     of this Agreement to use its best efforts to effect the registration of any
     Registrable Shares under the Securities Act, Aspen shall:

                     (i)    prepare and file with the Commission a Registration
                            Statement on an appropriate form as soon as
                            practicable, and cause such Registration Statement
                            to be declared effective by the Commission at the
                            earliest practicable date and in any event within 20
                            Business Days, provided that Aspen shall not be
                            required to file within such 20-Business Days if the
                            Registration Statement is required to include any
                            historical or pro forma financial statements not
                            previously filed by Aspen with the Commission;

                     (ii)   as expeditiously as possible prepare and file with
                            the Commission any amendments and supplements to the
                            Registration Statement and the Prospectus included
                            in the Registration Statement as may be necessary to
                            comply with the provisions of the Securities Act
                            (including the anti-fraud provisions thereof) and to
                            keep the Registration Statement effective for at
                            least 90 days from the effective date or such lesser
                            period until all such Registrable Shares are sold;

                     (iii)  as expeditiously as possible furnish to each Selling
                            Stockholder such reasonable numbers of copies of the
                            Prospectus, including any preliminary Prospectus, in
                            conformity with the requirements of the Securities
                            Act, and such other documents as such Selling
                            Stockholder may reasonably request in order to
                            facilitate the public sale or other disposition of
                            the Registrable Shares owned by such Selling
                            Stockholder;

                     (iv)   as expeditiously as possible use its best efforts to
                            register or qualify the Registrable Shares covered
                            by the Registration Statement under the securities
                            or Blue Sky laws of such states as the Selling
                            Stockholders shall reasonably request, and do any
                            and all other acts and things that may be necessary
                            or desirable to enable the Selling Stockholders to
                            consummate the public sale or other disposition in
                            such states of the Registrable Shares owned by the
                            Selling Stockholder, provided that Aspen shall not
                            be required in connection with this paragraph (iv)
                            to qualify as a foreign corporation or execute a
                            general consent to service of process in any
                            jurisdiction;

                     (v)    prior to the effective date of the Registration
                            Statement, cause all such Registrable Shares to be
                            listed on each securities exchange or automated or
                            inter-dealer quotation system on which similar
                            securities issued by Aspen are then listed;

                     (vi)   promptly provide a transfer agent and registrar for
                            all such Registrable Shares no later than the
                            effective date of such registration statement;

                     (vii)  promptly make available for inspection by the
                            Selling Stockholders, any managing underwriter
                            participating in any disposition pursuant to such
                            Registration Statement, and any attorney or
                            accountant or other agent retained by any such
                            underwriter or selected by the Selling Stockholders,
                            all financial and other records, pertinent corporate
                            documents and properties of Aspen and cause Aspen's
                            officers, directors, employees and independent
                            accountants to supply all

                                       5
<PAGE>   6
                            information reasonably requested by any such seller,
                            underwriter, attorney, accountant or agent in
                            connection with such Registration Statement;

                     (viii) notify each Selling Stockholder at any time when a
                            Prospectus relating to the Registration Statement is
                            required to be delivered under the Securities Act,
                            of the happening of any event as a result of which
                            the Prospectus included in such Registration
                            Statement, as then in effect, includes an untrue
                            statement of a material fact or omits to state a
                            material fact required to be stated therein or
                            necessary to make the statement therein not
                            misleading or incomplete in light of the
                            circumstances then existing, and promptly amend the
                            Registration Statement and/or related Prospectus to
                            correct such untrue statement or to include the
                            omitted information;

                     (ix)   as expeditiously as possible, notify each Selling
                            Stockholder, promptly after it shall receive notice
                            thereof, of the time when such Registration
                            Statement has become effective or a supplement to
                            any Prospectus forming a part of such Registration
                            Statement has been filed; and

                     (x)    as expeditiously as possible following the
                            effectiveness of such Registration Statement, notify
                            each seller of such Registrable Shares of any stop
                            order, order of formal or informal investigation or
                            any request by the Commission for the amending or
                            supplementing of such Registration Statement or
                            Prospectus.

                (b) Prospective Amendments. If Aspen has delivered a Prospectus
     to the Selling Stockholders and after having done so the Prospectus is
     amended to comply with the requirements of the Securities Act, Aspen shall
     promptly notify the Selling Stockholders and, if requested, the Selling
     Stockholders shall immediately cease making offers of Registrable Shares
     and return all Prospectuses to Aspen. Aspen shall promptly provide the
     Selling Stockholders with revised Prospectuses and, following receipt of
     the revised Prospectuses, the Selling Stockholders shall be free to resume
     making offers of the Registrable Shares.

         2.4. Allocation of Expenses. Aspen will pay all Registration Expenses
for all registrations under this Agreement; provided, however, that if a Demand
Registration under Section 2.1 is withdrawn at the request of the Initiating
Holders (other than as a result of information concerning the business or
financial condition of Aspen that is made known to the Stockholders after the
date on which such registration was requested) and if the Initiating Holders
elect not to have such registration counted as a registration requested under
Section 2.1(a), then the Initiating Holders shall pay the Registration Expenses
of such registration pro rata in accordance with the number of their Registrable
Shares included in the Demand Registration. For purposes of this Section, the
term "Registration Expenses" shall mean all reasonable expenses incurred in
effecting any registration pursuant to this Agreement, including all
registration and filing fees, exchange listing fees, printing expenses, fees and
expenses of counsel and accountants for Aspen and the fees and expenses of one
counsel selected by the Selling Stockholders to represent the Selling
Stockholders, state Blue Sky fees and expenses, and the expense of any regular
or special audits incident to or required by any such registration, but
excluding underwriting discounts, selling commissions and the fees and expenses
of Selling Stockholders' own counsel (other than the counsel selected to
represent all Selling Stockholders).

         2.5.   Indemnification and Contribution.

                (a) Indemnification by Aspen. In the event of any registration
     of any of the Registrable Shares under the Securities Act pursuant to this
     Agreement, Aspen will indemnify and hold harmless each Selling Stockholder,
     each underwriter of such Registrable Shares, and each other person, if any,

                                       6
<PAGE>   7
     who controls such Selling Stockholder or underwriter within the meaning of
     the Securities Act or the Exchange Act, with respect to each registration,
     qualification or compliance effected pursuant to this Section 2, against
     any losses, claims, damages, liabilities (or actions, proceedings or
     settlements in respect thereof), joint or several, to which such Selling
     Stockholder, underwriter or controlling person may become subject under the
     Securities Act, the Exchange Act, state securities or Blue Sky laws or
     otherwise, insofar as such losses, claims, damages or liabilities (or
     actions, proceedings or settlements in respect thereof) arise out of or are
     based upon any untrue statement or alleged untrue statement of any material
     fact contained in any Registration Statement under which such Registrable
     Shares were registered under the Securities Act, any preliminary prospectus
     or final prospectus contained in the Registration Statement, or any
     amendment or supplement to such Registration Statement, incident to any
     such registration, qualification or compliance, or arise out of or are
     based upon the omission or alleged omission to state a material fact
     required to be stated therein or necessary to make the statements therein
     not misleading; or any violation by Aspen of the Securities Act or Exchange
     Act or any rule or regulation thereunder applicable to Aspen and relating
     to action or inaction required by Aspen in connection with any such
     registration, qualification or compliance; and Aspen will reimburse such
     Selling Stockholder, underwriter and each such controlling person for any
     legal or any other expenses reasonably incurred by such Selling
     Stockholder, underwriter or controlling person in connection with
     investigating or defending any such loss, claim, damage, liability or
     action; provided, however, that Aspen will not be liable in any such case
     to the extent that any such loss, claim, damage or liability arises out of
     or is based upon any untrue statement or omission made in such Registration
     Statement, preliminary prospectus or prospectus, or any such amendment or
     supplement, in reliance upon and in conformity with written information
     furnished to Aspen, by or on behalf of such Selling Stockholder,
     underwriter or controlling person and stated to be specifically for use in
     the preparation thereof.

                (b) Indemnification by Selling Stockholders. In the event of any
     registration of any of the Registrable Shares under the Securities Act
     pursuant to this Agreement, each Selling Stockholder, severally and not
     jointly, will indemnify and hold harmless Aspen, each of its directors and
     officers and each underwriter (if any) and each person, if any, who
     controls Aspen or any such underwriter within the meaning of the Securities
     Act or the Exchange Act, against any losses, claims, damages or
     liabilities, joint or several, to which Aspen, such directors and officers,
     underwriter or controlling person may become subject under the Securities
     Act, Exchange Act, state securities or Blue Sky laws or otherwise, insofar
     as such losses, claims, damages or liabilities (or actions in respect
     thereof) arise out of or are based upon any untrue statement or alleged
     untrue statement of a material fact contained in any Registration Statement
     under which such Registrable Shares were registered under the Securities
     Act, any preliminary prospectus or final prospectus contained in the
     Registration Statement, or any amendment or supplement to the Registration
     Statement, or arise out of or are based upon any omission or alleged
     omission to state a material fact required to be stated therein or
     necessary to make the statements therein not misleading, if the statement
     or omission was made in reliance upon and in conformity with information
     relating to such Selling Stockholder furnished in writing to Aspen by or on
     behalf of such Selling Stockholder specifically for use in connection with
     the preparation of such Registration Statement, prospectus, amendment or
     supplement; provided, however, that the obligations of a Selling
     Stockholder hereunder shall be limited to an amount equal to the net
     proceeds to such Selling Stockholder of Registrable Shares sold in
     connection with such registration.

                (c) Indemnification Procedures. Each party entitled to
     indemnification under this Section (the "Indemnified Party") shall give
     notice to the party required to provide indemnification (the "Indemnifying
     Party") promptly after such Indemnified Party has actual knowledge of any
     claim as to which indemnity may be sought, and shall permit the
     Indemnifying Party to assume the defense of any such claim or any
     litigation resulting therefrom; provided, that counsel for the Indemnifying

                                       7
<PAGE>   8
     Party, who shall conduct the defense of such claim or litigation, shall be
     approved by the Indemnified Party (whose approval shall not be unreasonably
     withheld); and, provided, further, that the failure of any Indemnified
     Party to give notice as provided herein shall not relieve the Indemnifying
     Party of its obligations under this Section except to the extent that the
     Indemnifying Party is adversely affected by such failure. The Indemnified
     Party may participate in such defense at such party's expense; provided
     that the Indemnifying Party shall pay such expense if representation of
     such Indemnified Party by the counsel retained by the Indemnifying Party is
     determined to be inappropriate, based upon the advice of counsel, for the
     Indemnified Party due to actual or potential differing interests between
     the Indemnified Party and any other party represented by such counsel in
     such proceeding; provided, further, that in no event shall the Indemnifying
     Party be required to pay the expenses of more than one law firm per
     jurisdiction as counsel for the Indemnified Party. The Indemnifying Party
     also shall be responsible for the expenses of such defense if the
     Indemnifying Party does not elect to assume such defense. No Indemnifying
     Party, in the defense of any such claim or litigation shall, except with
     the consent of each Indemnified Party, consent to entry of any judgment or
     enter into any settlement that does not include as an unconditional term
     thereof the giving by the claimant or plaintiff to such Indemnified Party
     of a release from all liability in respect of such claim or litigation, and
     no Indemnified Party shall consent to entry of any judgment or settle such
     claim or litigation without the prior written consent of the Indemnifying
     Party, which consent shall not be unreasonably withheld.

                (d) Contribution. In order to provide for just and equitable
     contribution in circumstances in which the indemnification provided for in
     this Section 2.5 is due in accordance with its terms but for any reason is
     held to be unavailable to an Indemnified Party in respect to any losses,
     claims, damages and liabilities referred to herein, then the Indemnifying
     Party shall, in lieu of indemnifying such Indemnified Party, contribute to
     the amount paid or payable by such Indemnified Party as a result of such
     losses, claims, damages or liabilities to which such party may be subject
     in such proportion as is appropriate to reflect the relative fault of Aspen
     on the one hand and the Selling Stockholders on the other in connection
     with the statements or omissions that resulted in such losses, claims,
     damages or liabilities, as well as any other relevant equitable
     considerations. The relative fault of Aspen and the Selling Stockholders
     shall be determined by reference to, among other things, whether the untrue
     or alleged untrue statement of material fact related to information
     supplied (or required to be supplied) by Aspen or the Selling Stockholders
     and the parties' relative intent, knowledge, access to information and
     opportunity to correct or prevent such statement or omission. Aspen and the
     Selling Stockholders agree that it would not be just and equitable if
     contribution pursuant to this Section 2.5 were determined by pro rata
     allocation or by any other method of allocation that does not take account
     of the equitable considerations referred to above. Notwithstanding the
     provisions of this paragraph of Section 2.5, (a) in no case shall any one
     Selling Stockholder be liable or responsible for any amount in excess of
     the net proceeds received by such Selling Stockholder from the offering of
     Registrable Shares and (b) Aspen shall be liable and responsible for any
     amount in excess of such proceeds; provided that no person guilty of
     fraudulent misrepresentation (within the meaning of Section 11(f) of the
     Securities Act) shall be entitled to contribution from any person who was
     not guilty of such fraudulent misrepresentation. Any party entitled to
     contribution will, promptly after receipt of notice of commencement of any
     action, suit or proceeding against such party in respect of which a claim
     for contribution may be made against another party or parties under this
     Section, notify such party or parties from whom contribution may be sought,
     but the omission so to notify such party or parties from whom contribution
     may be sought shall not relieve such party from any other obligation it or
     they may have thereunder or otherwise under this Section. No party shall be
     liable for contribution with respect to any action, suit, proceeding or
     claim settled without its prior written consent, which consent shall not be
     unreasonably withheld.

                                       8
<PAGE>   9
         2.6. Other Matters with Respect to Underwritten Offerings. In the event
that Registrable Shares are sold pursuant to a Registration Statement in an
underwritten offering pursuant to Section 2.1, Aspen agrees to: (a) enter into
an underwriting agreement containing customary representations and warranties
with respect to the business and operations of Aspen and customary covenants and
agreements to be performed by Aspen, including without limitation customary
provisions with respect to indemnification by Aspen of the underwriters of such
offering; (b) use its best efforts to cause its legal counsel to render
customary opinions to the underwriters with respect to the Registration
Statement; and (c) use its best efforts to cause its independent public
accounting firm to issue a customary "cold comfort letter" to the underwriters
with respect to the Registration Statement.

         2.7. Information by Holder. Each Stockholder included in any
registration shall furnish to Aspen such information regarding such Stockholder
and the distribution proposed by such Stockholder as Aspen may reasonably
request in writing and as shall be required in connection with any registration,
qualification or compliance referred to in this Agreement.

         2.8. "Stand-Off" Agreement; Confidentiality of Notices. Each
Stockholder, if requested by Aspen and the managing underwriters of an
underwritten public offering by Aspen of Common Stock, shall not sell or
otherwise transfer or dispose of any Registrable Shares or other securities of
Aspen other than the securities covered by the registration statement held by
such Stockholder for the applicable lock-up period under lock-up arrangements
generally entered into by selling stockholders, executive officers and directors
of Aspen at the request of such managing underwriters, provided that such period
shall not extend beyond 180 days after the date of the final prospectus for such
offering. Aspen may impose stop-transfer instructions with respect to the
Registrable Shares or other securities subject to the foregoing restriction
until the end of such 180-day period. Any Stockholder receiving any written
notice from Aspen regarding Aspen's plans to file a Registration Statement shall
treat such notice confidentially and shall not disclose such information to any
person other than as necessary to exercise his rights under this Agreement.

         2.9.   Rule 144 Requirements.  Aspen agrees to:

                (a) make and keep current public information about Aspen
                    available, as those terms are understood and defined in Rule
                    144;

                (b) use its best efforts to prepare and file with the Commission
                    in a timely manner all reports and other documents required
                    of Aspen under the Securities Act and the Exchange Act (at
                    any time after it has become subject to such reporting
                    requirements); and

                (c) promptly furnish to any holder of Registrable Shares upon
                    request (A) a written statement by Aspen as to its
                    compliance with the reporting requirements of Rule 144 and
                    of the Securities Act and the Exchange Act (at any time
                    after it has become subject to such reporting requirements),
                    (B) a copy of the most recent annual or quarterly report of
                    Aspen, and (C) such other reports and documents of Aspen as
                    such holder may reasonably request to avail itself of any
                    similar rule or regulation of the Commission allowing it to
                    sell any such securities without registration.

         2.10. Sections 2.1 and 2.2 Termination. All of Aspen's obligations to
register Registrable Shares under Sections 2.1 and 2.2 shall terminate on the
earlier of (a) the second anniversary of the date of this Agreement and (b) the
first date after the first anniversary of the date of this Agreement any date on
which (i) all of the Registrable Shares of such Stockholder may be sold within a
three-month period pursuant to Rule 144 and (ii) the number of Registrable
Shares held by such Stockholder is less than the limitation set forth in
paragraph (e)(1)(i) of Rule 144.

                                       9
<PAGE>   10
3.   Transfers of Rights

         This Agreement, and the rights and obligations of each Stockholder
hereunder, may be assigned by such Stockholder to any person or entity to which
at least 25,000 Shares are transferred by such Stockholder, and such transferee
shall be deemed a "Stockholder" for purposes of this Agreement; provided that
the transferee provides written notice of such assignment to Aspen and agrees in
writing to be bound hereby.

4.   General

         4.1. Amendments and Waivers. Any term of this Agreement may be amended
or terminated and the observance of any term of this Agreement may be waived
(either generally or in a particular instance and either retroactively or
prospectively), with the written consent of Aspen and the holders of at least a
majority of the Registrable Shares held by all of the Stockholders. Any such
amendment, termination or waiver effected in accordance with this Section 4.1
shall be binding on all parties hereto, even if they do not execute such
consent. No waiver by any party hereto with respect to any condition or breach
hereunder shall be deemed to extend to any prior or subsequent condition or
breach hereunder or affect in any way any rights arising by virtue of any prior
or subsequent condition or breach. No failure of the part of any parties hereto
to exercise, and no delay in exercising any right hereunder shall operate as a
waiver thereof; nor shall any single or partial exercise of any such right
preclude any other or further exercise thereof or the exercise of any other
right.

         4.2.   Construction.

                (a) The section headings contained in this Agreement are
     inserted for convenience only and shall not affect in any way the meaning
     or interpretation of this Agreement.

                (b) The language used in this Agreement shall be deemed to be
     the language chosen by the parties hereto to express their mutual intent,
     and no rule of strict construction shall be applied against a party hereto.

                (c) The term "including" as used herein shall not be construed
     so as to exclude any other thing not referred to or described.

                (d) References herein to "Sections" shall be deemed to be to
     sections of this Agreement, unless otherwise specified.

         4.3. Entire Agreement; Successors. This Agreement (a) constitutes the
entire agreement, and supersedes all other prior agreements and understandings,
both written and oral, between the parties with respect to the subject matter
hereof, and (b) is not intended to confer upon any person other than the parties
hereto any rights or remedies hereunder, except as otherwise expressly provided
herein. Subject to the preceding sentence, this Agreement shall be binding upon
and inure to the benefit of the parties named herein and their respective
successors and permitted assigns.

         4.4. Governing Law. This Agreement shall be governed by and construed
in accordance with the internal laws of the Commonwealth of Massachusetts
without giving effect to any choice or conflict of law provision or rule
(whether of the Commonwealth of Massachusetts or any other jurisdiction) that
would cause the application of laws of any jurisdictions other than those of the
Commonwealth of Massachusetts. THE PARTIES HERETO WAIVE ANY RIGHT THEY MAY HAVE,
AND AGREE NOT TO DEMAND, A TRIAL BY JURY.

                                       10
<PAGE>   11
         4.5. Notices. All notices, instructions, demands, claims, requests and
other communications given hereunder or in connection herewith shall be in
writing. Any such communication shall be sent either (a) by registered or
certified mail, return receipt requested, postage prepaid, or (b) via a
reputable nationwide overnight courier service, in each case to the address set
forth below. Any such communication shall be deemed to have been delivered two
business days after it is sent by registered or certified mail, return receipt
requested, postage prepaid, or one business day after it is sent via a reputable
nationwide overnight courier service.

         To Aspen                       Aspen Technology, Inc.
                                        Ten Canal Park
                                        Cambridge, Massachusetts  02141
                                        Facsimile:  617.577.0722
                                        Attention:  Chief Executive Officer

              With a copy to:           Hale and Dorr LLP
                                        60 State Street
                                        Boston, Massachusetts  02109
                                        Facsimile:  617.526.5000
                                        Attention:  Mark L. Johnson

         To any Stockholder:            At such Stockholder's address of
                                        record in the stock transfer records
                                        of Aspen

              With a copy to:           Elias, Matz, Tiernan & Herrick L.L.P.
                                        734 15th Street, N.W.
                                        Washington, D.C.  20005
                                        Facsimile:  202.347.2172
                                        Attention:  Jeffrey A. Koeppel

Any party hereto may give any notice, instruction, demand, claim, request or
other communication hereunder using any other means (including personal
delivery, expedited courier, messenger service, telecopy, telex, ordinary mail
or electronic mail), but no such communication shall be deemed to have been duly
given unless and until it actually is received by the party for which it is
intended. Any party hereto may change the address to which notices,
instructions, demands, claims, requests and other communications hereunder are
to be delivered by giving the other parties hereto notice in the manner set
forth in this Section 4.5.

         4.6. Severability. Any term or provision of this Agreement that is
invalid or unenforceable in any circumstances in any jurisdiction shall not
affect the validity or enforceability of the remaining terms and provisions
hereof or the validity or enforceability of the offending term or provision in
any other circumstances or in any other jurisdiction. If the final judgment of a
court of competent jurisdiction declares that any term or provision hereof is
invalid or unenforceable, the parties hereto agree that the court making the
determination of invalidity or unenforceability shall have the power to limit
the term or provision, to delete specific words or phrases, or to replace any
invalid or unenforceable term or provision with a term or provision that is
valid and enforceable and that comes closest to expressing the intention of the
invalid or unenforceable term or provision, and this Agreement shall be
enforceable as so modified.

         4.7. Signatures. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument. This Agreement may be
executed by facsimile signature.

                                       11
<PAGE>   12
     IN WITNESS WHEREOF, the parties have executed this Agreement as of the date
first set forth above.

                                        ASPEN TECHNOLOGY, INC.

                                        By:      /s/ Mary A. Palermo
                                        ----------------------------------------
                                             Title:  Executive Vice President

                                        STOCKHOLDERS:

                                             /s/ Eunice E. Blecker
                                        ----------------------------------------
                                        EUNICE E. BLECKER

                                             /s/ Herbert G. Blecker
                                        ----------------------------------------
                                        HERBERT G. BLECKER

                                       12

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