<Page>

                                                                     EXHIBIT 2.1


<Page>

================================================================================

                        --------------------------------

                         BUSINESS COMBINATION AGREEMENT

                        --------------------------------
                        dated as of 30/31 December, 2000

                                      among

                        MESSER INDUSTRIEGESELLSCHAFT MBH,
                             MESSER GRIESHEIM GMBH,
                      CORNELIA VERWALTUNGSGESELLSCHAFT MBH,
                           HOECHST AKTIENGESELLSCHAFT
                                       AND
                               DIOGENES ZWANZIGSTE
                            VERMOGENSVERWALTUNGS GMBH

                        --------------------------------

                                   [GRAPHIC]

================================================================================

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                                       ii

                               TABLE OF CONTENTS
<Table>
                                                                        
1.  Certain Definitions .....................................................2
2.  Transactions Forming the Business Combination ...........................9
    2.1   Certain Dates .....................................................9
    2.2   MGG's Pre-Closing Restructuring ..................................10
    2.3   [Intentionally left blank] . .....................................10
    2.4   Contribution and Transfer of Hoechst-MGG Share to Hoechst
          NewCo II .........................................................10
    2.5   Sale of China Interests ..........................................11
    2.6   Share Loan of Hoechst NewCo II Shares to Hoechst NewCo III .......13
    2.7   Sale of Hoechst NewCo II Shares and Hoechst NewCo II
          Contribution Receivable to Investor NewCo . ......................14
    2.8   Call Option for Hoechst NewCo II Option Shares ...................16
    2.9   Counter-Call Option for Hoechst NewCo III Shares .................18
    2.10  Counter-Put Option for Hoechst NewCo III Shares ..................19
    2.11  Summary of Steps .................................................20
    2.12  Closing Transactions .............................................22
3.  Conditions to Closing ..................................................23
    3.1   Conditions to the Parties' Obligations ...........................23
    3.2   Non-Fulfillment of Closing Conditions ............................27
    3.3   Exemption of Certain Transaction .................................27
    3.4   MAC Determination and Breach of Rep Determination ................28
4.  Covenants...............................................................30
    4.1   General...........................................................30
    4.2   Covenants of Hoechst and MIG......................................31
    4.3   Negative Covenants of Hoechst.....................................32
    4.4   Hoechst Nominees..................................................32
    4.5   Access and Information............................................32
    4.6   Financial Statements 2000.........................................32
    4.7   Business Year.....................................................33
    4.8   Covenants of Investor.............................................34
    4.9   Antitrust Regulatory Approvals....................................34
    4.10  Pensionskasse in Germany .........................................36
    4.11  [intentionally left blank]........................................36
    4.12  Covenants of Investor NewCo ......................................36
    4.13  Covenants under Contribution Agreements ..........................39
    4.14  Non-Fulfillment of Counter-Call ..................................40
    4.15  No Change of Forward Sale Agreements .............................41
    4.16  Special Indemnification ..........................................41

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                                     -iii-

    4.17  Disproportionate Guarantee Indemnification . .....................43
    4.18  Monetarization of Loan Notes .....................................51
    4.19  Singapore ........................................................51
    4.20  Contributions by Investor NewCo ..................................52
    4.21  Additional Hoechst Shareholding in Messer Egypt S.A.E ............52
5.  Representations and Warranties of Hoechst ..............................53
    5.1   Messer Group......................................................53
    5.2   Capital Stock.....................................................54
    5.3   No Breach.........................................................54
    5.4   No Encumbrances on Stock .........................................55
    5.5   Financial Statements .............................................55
    5.6   Ordinary Course Operation ........................................56
    5.7   Real Property. ...................................................56
    5.8   Gas Business Assets ..............................................57
    5.9   Intellectual Property Rights .....................................58
    5.10  Contracts and Commitments . ......................................58
    5.11  Litigation .......................................................61
    5.12  Employees ........................................................62
    5.13  Employee Benefit Plans ...........................................62
    5.14  Capital Contributions/Distributions ..............................64
    5.15  Compliance with Laws; Permits; Certain Operations ................64
    5.16  Insurance ........................................................64
    5.17  Other Liabilities ................................................65
    5.18  Finance Leases ...................................................65
    5.19  Consolidated Debt at Year End ....................................65
    5.20  Special Rep ......................................................67
6.  Indemnification By Hoechst .............................................67
    6.1   General ..........................................................67
    6.2   Loss .............................................................68
    6.3   Hoechst Damage Quota, De Minimis Amount, Threshold and Cap .......68
    6.4   Third-Party-Claims ...............................................70
    6.5   Waiver of Claims .................................................71
    6.6   Limitation .......................................................71
    6.7   Mitigation .......................................................72
    6.8   Indemnification of Hoechst .......................................73
    6.9   Sole Remedy ......................................................74
    6.10  Exclusion of Liability ...........................................75
    6.11  Oral Statements ..................................................75
    6.12  Knowledgeable Persons ............................................76
    6.13  Exclusion of Sections 439, 460, 464 ..............................76

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                                      -iv-

    6.14  Aventis China Companies ..........................................76
    6.15  Audit Review......................................................76
    6.16  Specific Remedy For Section 5.20..................................78
7.  Representations and Warranties of Investor NewCO .......................78
    7.1   Representations ..................................................78
    7.2   Indemnification...................................................79
8.  Tax Matters ............................................................79
    8.1   Tax Indemnities ..................................................79
    8.2   Refunds and Benefits .............................................82
    8.3   Preparation of Tax Returns .......................................84
    8.4   Cooperation and Exchange of Information ..........................84
    8.5   Notification .....................................................85
    8.6   Contest ..........................................................86
    8.7   Conveyance Taxes .................................................87
    8.8   Miscellaneous . ..................................................87
    8.9   Clarifications ...................................................88
    8.10  Aventis China Companies ..........................................88
9.  Environmental Matters ..................................................89
    9.1   Refund and Allocation of Damage Control Expense ..................89
    9.2   Conditions to Certain Refunds ....................................91
    9.3   Post-Closing Environmental Contamination .........................94
    9.4   Application for Refund ...........................................95
    9.5   Aventis China Companies ..........................................96
10. Termination ............................................................96
    10.1  Termination ......................................................96
    10.2  Special Termination Right of Investor NewCo ......................97
    10.3  Special Termination Right of Hoechst .............................98
    10.4  Effect of Termination ............................................99
11. Guarantees .............................................................99
12. Additional Agreements ..................................................99
    12.1  MIG Rights .......................................................99
    12.2  Non-Compete. ....................................................100
    12.3  Expenses and Fees ...............................................101
    12.4  Notices..........................................................101
    12.5  Amendments.......................................................102
    12.6  Headings.........................................................103
    12.7  Default Interest Rate ...........................................103
    12.8  Rights of Retention .............................................103
    12.9  Complete Agreement ..............................................103
    12.10 Successors ......................................................104

</Table>

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                                      -v-
<Table>

                                                                        
    12.11 Counterparts ....................................................104
    12.12 Confidentiality .................................................105
    12.13 Severability Clause .............................................106
    12.14 Access to Information ...........................................106
    12.15 Governing Law  ..................................................106
    12.16 Arbitration  ....................................................107
</Table>


                                LIST OF EXHIBITS*

Exhibit A             Aventis China Companies
Exhibit B             List of Material Messer Companies
Exhibit C             List of Messer Group Companies
Exhibit D             Certain Investigations
Exhibit E             Certain Activities
Exhibit 2.2           Foreign Sub Contribution Agreement
Exhibit 2.4           Hoechst NewCo II Contribution Agreement
Exhibit 2.5           China Sale and Purchase Agreement
Exhibit 2.6           Share Loan Agreement
Exhibit 2.7(a)(1)     Hoechst NewCo II Share Purchase Agreement I
Exhibit 2.7(b)        Hoechst NewCo II Contribution Receivable Purchase
                      Agreement
Exhibit 2.8(a)        Hoechst NewCo II Share Purchase Agreement II
Exhibit 2.8(b)        Management Certificate before Exercise Call Option
Exhibit 2.9(a)        Hoechst NewCo III Share Purchase Counter-Call Agreement
Exhibit 2.9(b)        Form of Acceptance Declaration (for executing Counter-Call
                      Option)
Exhibit 2.10(a)       Hoechst NewCo III Share Purchase Counter-Put Agreement
Exhibit 2.10(b)       Form of Acceptance Declaration (for executing Counter-Put
                      Option)
Exhibit 3.1(b)(i)     Forward Sale Agreement I
Exhibit 3.1(b)(ii)    Forward Sale Agreement II
Exhibit 3.1(h)        Companies not to become insolvent

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                                      -vi-

Exhibit 3.1(j)        Aventis Guarantee
Exhibit 3.1(l)        Singapore Separation Agreement ("Formation, Funding and
                      Shareholders' Agreement")
Exhibit 4.2           Instruction Letter
Exhibit 4.6           Write-Off Schedule
Exhibit 4.8           Equity Commitment Letters
Exhibit 4.12(a)       New Articles of Hoechst NewCo II
Exhibit 4.12(h)       Financial Information Investor NewCo
Exhibit 4.12          Possible Restructuring of Hoechst NewCo II
Exhibit 4.17          Disproportionate Guarantee Schedule
Exhibit 5             Disclosures
Exhibit 9.1           Table 1: 5 environmental sites
                      Table 2: Other disclosed contamination

*The following exhibits have been omitted from this filing, but will be provided
to the SEC upon its request.


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                                       1

                             INDEX OF DEFINED TERMS
<Table>
                                           
10 Business Days Delay Rule ...................45
2002 Break-Up Situation .......................38
66% Principle .................................64
Acetylene Companies ............................2
ACIC ...........................................2
Adjusted Excess Percentage ....................42
Affiliate ......................................2
Agreement ......................................2
Ancillary Agreements ...........................2
Antitrust Approvals ...........................22
Antitrust Filings .............................32
Applicable Law .................................2
Applicable Rate ...............................45
Aventis ........................................3
Aventis China Companies ........................3
Aventis Credit Drop ...........................16
Bank Sub ......................................17
Business Day ...................................3
Call Option ...................................15
Call Option Period ............................15
Call Option Price .............................15
China Drop Date ................................3
China Interests ................................3
China Limitation Date .........................67
China Purchase Price ..........................10
China Purchase Price Guarantee ................11
China Sale and Purchase Agreement .............10
China SPA .....................................10
Closing ........................................3
Closing Date ...................................9
Closing Drop Date .............................90
Closing Time ..................................21
Combination Documents ..........................3
Company ........................................1
Company Shares .................................1
Consolidated Messer Companies ..................3
Consultants ...................................58
Contest .......................................80
Contractual Interest Rate ......................3
Controlled Covered Entity ......................3
Corporate Documents ...........................51
Counter Option Price ..........................17
Counter-Call Option ...........................16
Counter-Call Option Period ....................17
Counter-Put Option ............................18
Counter-Put Option Period .....................18
Counter-Put Option Price ......................18
Covered Entities ..............................41
Covered Guarantee .............................41
Covered Period ................................43
Damage Control Expenses ........................4
Damage Control Measures ........................4
date hereof ....................................4
De Minimis Amount .............................65
Deferred Purchase Price .......................13
Definitive Financing Commitment ...............91
Disproportionate Guarantee Schedule ...........41
Due Diligence...................................1
Encumbrances ..................................52
Environmental Cap .............................85
Environmental Companies ........................4
Environmental Contamination ....................4
Environmental Damage ...........................4
Environmental De Minimis Amount ...............84
Environmental Indemnification Companies ........5
Environmental Law ..............................5
Environmental Threshold .......................84
EU Regulation .................................22
Excess Percentage .............................42
Expert ........................................86
Final MAC Determination .......................27
Financial Date .................................9
Financial Date Exposure .......................42
Financial Debt .................................5
Financial Guarantee ............................5
Financial Statements 1999 ......................5
Financial Statements 2000 ......................5
Financing .....................................91
Firm ..........................................72
Foreign Step-up ............................... 9
Foreign Sub Contribution Agreement ............ 9
Former Messer Real Estate .....................84
Forward Sale Agreement I ......................21
Forward Sale Agreement II .....................21
Fully Owned ................................... 5
Fund ..........................................73
Gas Business .................................. 1
German Participating Employees ................34
Governmental Entity ........................... 5
Governmental Orders ...........................85
GS Funds ......................................32
Guarantee Cap .................................66
Guarantee Demand Notice .......................43
Guarantee Holder ..............................41
Guarantee Percentage ..........................42
Hoechst ....................................... 5
Hoechst Damage Control Expense Quota ..........83
Hoechst Damage Quota ..........................64
Hoechst Group ..................................6
Hoechst NewCo II ...............................6
Hoechst NewCo II Contribution Agreement .......10
Hoechst NewCo II Contribution Receivable ......10
Hoechst NewCo II Option Shares .................6
Hoechst NewCo II Receivable Purchase Price ....13
Hoechst NewCo II Share Purchase Agreement I ...13

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                                      -2-

Hoechst NewCo II Share Purchase Agreement II ..15
Hoechst NewCo II Shares ........................6
Hoechst NewCo III ..............................6
Hoechst NewCo III Share Purchase Counter-
Call Agreement ................................17
Hoechst NewCo III Share Purchase Counter-Put
Agreement .....................................18
Hoechst NewCo III Shares .......................6
Hoechst Payment Amount ........................44
Hoechst-MGG Share ..............................1
HSR Act .......................................22
IAS ............................................6
Improvements ..................................53
Income Tax ....................................81
Indemnification Period ........................39
Insolvency ....................................23
Insolvent .....................................23
Interests .....................................50
Investor NewCo .................................6
Key Employees .................................58
Lien ...........................................6
Loan Notes ....................................13
Loss ..........................................64
MAC Determination Notice ......................26
MAC Dispute Notice ............................26
Main MGG Purchase Price .......................13
Material Messer Companies ......................6
Material Messer Contracts .....................57
Material Messer Owned Real Property ...........53
Maximum Guaranteed Amount .....................42
Messer Capital Stock ..........................51
Messer Foreign Holding .........................6
Messer Group ...................................6
Messer Group Companies .........................7
Messer Real Estate .............................7
MGG ............................................1
MGG Designee ...................................7
MGG Guarantor .................................41
MGG Payment Date ..............................43
MGG Percentage ................................42
MIG.............................................1
MIG-MGG Share ..................................1
Minimum Damage Amount .........................37
Non-Fully Owned ................................7
Objection Notice ..............................72
Other Non-Controlled Covered Entity ............7
Participations ................................50
Payment Notice ................................43
Permits ........................................7
Permitted Liens ................................7
Person .........................................7
Plans .........................................59
Pre-Transaction Distributed Capital Situation..67
Pro Rata Exception Principle ..................65
Pro Rata Principle ............................64
Pro Rata Shareholding Principle ...............64
RAP ...........................................87
Recontribution Note ...........................36
Reference Deed .................................2
Reimbursement Application .....................89
Share Loan Agreement ..........................12
Singapore Separation Agreement ................24
Site ...........................................7
Sold Covered Entity ............................7
Special Foreign Companies ......................7
Special Non-Controlled Covered Entity ..........7
Standstill Period .............................46
Subsidiary .....................................7
Tax ............................................8
Tax Authority ..................................8
Tax Indemnification Company ....................8
Tax Return .....................................8
Threshold .....................................65
Total Compensation .............................8
Total Exposure ................................42
Total MGG Payment .............................43
Write-Downs ...................................30
</Table>

<Page>

                                   WITNESSETH

         WHEREAS, Messer Griesheim GmbH (the "COMPANY" or "MGG") is engaged,
directly and through subsidiaries, in various business activities, including
without limitation the business of developing, producing and marketing
industrial gases (all of such business activities other than activities related
to the cutting and welding business which has been or agreed to be sold to MIG,
collectively the "GAS BUSINESS").

         WHEREAS, MGG is a limited liability company organized and existing
under the laws of the Federal Republic of Germany, registered with the
commercial register of the Amtsgericht Frankfurt am Main under No. HR B 7812,
and its stated share capital amounts to DM 540,000,000 (in words: Deutsche Mark
five hundred and forty million) which is represented as of the date hereof by
two (2) shares as follows: one (1) share with a nominal value of DM 360,000,000
is held by Hoechst (the "HOECHST-MGG SHARE") and one (1) share with a nominal
value of DM 180,000,000 is held by certain members of the Messer family through
the Messer Industrie GmbH ("MIG") (the "MIG-MGG SHARE" and together with the
Hoechst MGG Share, the "COMPANY SHARES").

         WHEREAS, Hoechst is interested in having a right to sell its interest
in the Gas Business and to sell shares in certain Chinese companies, and
Investor NewCo is interested in buying such interest and MGG in buying such
shares in the Chinese companies.

         WHEREAS, MIG is interested in remaining active in the Gas Business and
in accepting Investor NewCo as a new shareholder of the Company.

         WHEREAS, Investor NewCo had the opportunity to conduct a due diligence
with respect to substantive matters of the Messer Group Companies prior to the
notarization of this Agreement (the "DUE DILIGENCE").

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants contained herein, the parties hereto, intending to be legally bound,
hereby agree as follows:

<Page>

                                      -2-


INTRODUCTORY REMARKS

At certain places in this Agreement reference is made to a "Reference Deed"
which deed shall mean the notarial deed of notary public Dr. Peter Gamon in
Frankfurt (Ur.-Nr. G 679/2000 (the "REFERENCE DEED"). In this Reference Deed,
Ms. Silvia Marxen acted as a representative for all parties of this Agreement
without power of attorney. The parties hereby approve all declarations made by
Ms. Marxen and hereby confirm that they have full knowledge of the content of
the Reference Deed. Furthermore, the parties hereby incorporate such Reference
Deed into this Agreement and waive their right to have such Reference Deed read
aloud and attached hereto. The Reference Deed was available for inspection
during the notarization of this Agreement. Wherever in this Agreement reference
is made to Exhibits, and these Exhibits are not attached hereto, the reference
is made to the Exhibits of the Reference Deed.

1.       CERTAIN DEFINITIONS

The following terms shall have the following meanings:

"ACETYLENE COMPANIES" means Messer Mostar Plin d.o.o., Compania de Productos
(COPA 3091), Gases Industriales S.A. de C.V., Messer Namibia (Propietary)
Ltd., Messer Energo Gaz S.R.L., Messer Industrial Gases (Private) Limited,
Industrial Gases (Trinidad), Messer Haiphong (Vietnam) Industrial Gases Ltd.

"ACIC" means Aventis (China) Investment Co. Ltd.

"AFFILIATE" means, with respect to any Person, any other Person that directly or
indirectly through one or more intermediaries controls, is controlled by, or is
under common control with such Person.

"AGREEMENT" means this Business Combination Agreement including all Exhibits
thereto.

"ANCILLARY AGREEMENTS" means the following agreements including all Exhibits
thereto: the Hoechst NewCo II Contribution Agreement, the Forward Sale Agreement
I, the Forward Sale Agreement II and the Share Loan Agreement.

"APPLICABLE LAW" means any statute, law, ordinance, rule, regulation or code of
any Governmental Entity applicable to a specific legal entity or situation.

<Page>

                                      -3-


"AVENTIS" means Aventis, a stock company under French law with a supervisory
board and its registered offices in Schiltigheim, France.

"AVENTIS CHINA COMPANIES" means the companies listed in EXHIBIT A (and being
identical with the companies defined as ACIC Gas Companies in the China Sale and
Purchase Agreement).

"BUSINESS DAY" means any day that is not a Saturday, Sunday or any other day on
which banks are required by Applicable Law to be closed in the State of Hessen,
Federal Republic of Germany.

"CHINA DROP DATE" means the first day following the expiry of a period of
twenty-four (24) months after the Closing Date.

"CHINA INTERESTS" means the shareholding of ACIC in the Aventis China Companies
(and being identical with the ACIC Gas Interests as defined in the China Sale
and Purchase Agreement).

"CLOSING" means the consummation of the transactions to be performed at the
Closing Date.

"COMBINATION DOCUMENTS" means this Agreement including its Exhibits, the
Ancillary Agreements, the China SPA, the Singapore Separation Agreement, the
Hoechst NewCo II Share Purchase Agreement I, the Hoechst NewCo II Share Purchase
Agreement II, the Hoechst NewCo III Share Purchase Counter-Call Agreement, the
Hoechst NewCo III Share Purchase Counter-Put Agreement, the Foreign Sub
Contribution Agreement and the Hoechst NewCo II Contribution Receivable Purchase
Agreement, all including all Exhibits thereto.

"CONSOLIDATED MESSER COMPANIES" means MGG and the Messer Group Companies which
have been fully consolidated in the Financial Statements 1999.

"CONTRACTUAL INTEREST RATE" means, in respect of any calendar quarters, interest
at a rate of EURIBOR (six months) + 100 basis points as per the first banking
day of that calendar quarter in which the respective interest period starts,
adjusted every new calendar quarter after such first banking day to the then
prevailing EURIBOR (six months), and ending on the day on which payment is
received by the other party, and which interest shall be calculated on the basis
of the number of days elapsed during such period divided by 360 to be compounded
semiannually.

"CONTROLLED COVERED ENTITY" means the entities described as such in EXHIBIT
4.17.

<Page>

                                      -4-


"DAMAGE CONTROL EXPENSES" means (i) costs and damages incurred or payments made
after the Financial Date to implement a Damage Control Measure, including
without limitation the technical and analytical costs, excavations,
transportation, storage and treatment of polluted soil and groundwater, and (ii)
costs and expenses, including fees, disbursements and expenses of legal counsel,
experts and engineers related to the foregoing and incurred after the Financial
Date; PROVIDED that such costs and damages under (i) are kept as low as possible
consistent with applicable requirements of Environmental Law and Permits, and
such costs under (ii) are commercially reasonably incurred. Expressly excluded
are (i) internal administrative and overhead costs of implementation of the
Damage Control Measures and (ii) damages resulting from a business interruption
to the extent covered by insurances in force as of the Financial Date which
insurances shall be deemed to remain in force indefinitely. In the event that
a Damage Control Measure is undertaken as part of a construction project, only
the additional expenses incurred in connection with such construction project on
account of the Environmental Contamination shall be considered a Damage Control
Expense.

"DAMAGE CONTROL MEASURES" means any measures (including penalties and damage
payments to third parties) undertaken by any of the Environmental Companies
arising from an Environmental Damage.

"DATE HEREOF" means September 5, 2000.

"ENVIRONMENTAL COMPANIES" means the Environmental Indemnification Companies and
the Acetylene Companies.

"ENVIRONMENTAL CONTAMINATION" means any pollutants, contaminants or other
substances (including liquids, solids, gases) that are present in or on
buildings, surface water, soil air, soil or ground water or emitted from the
respective real property, that, pursuant to any Environmental Law applicable at
the location of the Environmental Contamination, are required to be remediated.

"ENVIRONMENTAL DAMAGE" means any liability or obligation of (i) an Environmental
Indemnification Company arising from any Environmental Contamination that (A)
exists on the Messer Real Estate on the Closing Date, or (B) has migrated from
the Messer Real Estate on or prior to the Closing Date, or (C) has otherwise
been caused by an Environmental Indemnification Company on or prior to the
Closing Date, or (ii) any of the Acetylene Companies arising from an
Environmental Contamination to the extent it relates to the current or former
production of acetylene, including purification and cleaning or maintenance of
the machinery, other equipment and transportation vehicles

<Page>

                                      -5-


or containers used therefor and the disposal of wastes resulting therefrom, and
that (A) exists on the Closing Date on the real estate owned or otherwise
occupied by an Acetylene Company on the Closing Date, or (B) has migrated from
such real estate on or prior to the Closing Date, or (C) has otherwise been
caused by an Acetylene Company on or prior to the Closing Date.

"ENVIRONMENTAL INDEMNIFICATION COMPANIES" means the Material Messer Companies,
all Messer Group Companies which have their registered seat in India,
Industrial Gases Ltd. (Trinidad), Messer Spain S.A. and Sauerstoffvertrieb
Wilhelm Geldbach GmbH & Co. KG.

"ENVIRONMENTAL LAW" means all federal, state and local German and non-German
statutes, regulations, ordinances, Permits and similar provisions, including
standards and common law having the force or effect of law, concerning as
legislative objective or in the individual case the protection of health or
pollution or protection of the environment, as such laws are in effect prior to
or on the Closing Date, and all judicial and administrative orders and
determinations based on such laws.

"FINANCIAL DEBT" means any interest bearing payment obligations to banks or
similar financial institutions (Section 266 III C No. 2 HGB - German
Commercial Code).

"FINANCIAL GUARANTEE" means a guarantee or similar enforceable contractual
obligation given by a Person to secure the Financial Debt of another Person.

"FINANCIAL STATEMENTS 1999" means the consolidated audited IAS financial
statements of the Company for the business year ending on December 31, 1999.

"FINANCIAL STATEMENTS 2000" means the consolidated financial statements of the
Company and the individual financial statements of the Consolidated Messer
Companies for the business year ending on December 31, 2000.

"FULLY OWNED" used with respect to any Messer Group Company means such company
which is directly or indirectly 100% owned by MGG.

"GOVERNMENTAL ENTITY" means any foreign or domestic government, agency,
governmental department, commission, board, bureau, court or instrumentality or
any state or other political subdivision thereof (whether now or hereafter
constituted and/or existing) and any entity exercising executive, legislative,
judicial, regulatory or administrative functions of or pertaining to
government.

"HOECHST" means Hoechst Aktiengesellschaft.

<Page>

                                      -6-


"HOECHST GROUP" means Hoechst, Aventis and any of their Affiliates other than
the Messer Group.

"HOECHST NEWCO IV" means DIOGENES Vierte Vermogensverwaltungs
Aktiengesellschaft, a German stock corporation (AKTIENGESELLSCHAFT) with a
registered share capital of DM 100,000, to be converted and increased to EUR
60,000 prior to Closing, registered with the commercial register of AG Frankfurt
am Main under HR B 42291 and being as of the date hereof a wholly owned
subsidiary of Hoechst.

"HOECHST NEWCO III" means DIOGENES Zwanzigste Vermogensverwaltungs GmbH, a
German limited liability company (GmbH) with a registered capital of EUR 25,000
registered with the commercial register of Frankfurt am Main under HR B 48032
and being as of the date hereof a wholly owned subsidiary of Hoechst.

"HOECHST NEWCO II OPTION SHARES" means 60,000 shares in Hoechst NewCo II with no
par value, each share giving a participation right of EUR I and being identical
with the Hoechst NewCo II Shares.

"HOECHST NEWCO II SHARES" means all of the issued and outstanding shares in
Hoechst NewCo II as of the Closing Date.

"HOECHST NEWCO III SHARES" means all of the issued and outstanding shares in
Hoechst NewCo III as of the date when such shares are transferred to Investor
NewCo upon exercise of the Counter-Call Option or Counter-Put Option.

"IAS" means International Accounting Standards in effect as of the date of the
related financial statements.

"INVESTOR NEWCO" means CORNELIA Verwaltungsgesellschaft mbH, a German limited
liability company (GmbH) with a registered capital of EUR 25,000 registered with
the commercial register Frankfurt am Main under HRB 50040.

"LIEN" means any lien, pledge, mortgage, deed of trust, security interest,
charge, option, right of first refusal, easement, servitude, transfer
restriction or other similar limitation whatsoever.

"MATERIAL MESSER COMPANIES" means the Companies listed in EXHIBIT B.

"MESSER FOREIGN HOLDING" means Messer International GmbH, a wholly owned
subsidiary of MGG.

"MESSER GROUP" means all the Messer Group Companies together.

<Page>

                                      -7-


"MESSER GROUP COMPANIES" means MGG, Messer Foreign Holding and the companies
listed in EXHIBIT C.

"MESSER REAL ESTATE" means Material Messer Owned Real Property and all real
estate that is on or was prior to the Closing Date occupied by the Messer Group
Companies or their predecessors.

"MGG DESIGNEE" has the meaning given to such term in the China SPA.

"NON-FULLY OWNED" used with respect to any Messer Group Company means any such
company which is neither directly nor indirectly Fully Owned.

"OTHER NON-CONTROLLED COVERED ENTITY" means the entities described as such in
EXHIBIT 4.17.

"PERMITS" means permits, licenses, certificates, accreditations or other
authorizations or consents of Governmental Entities.

"PERMITTED LIENS" means with respect to any real estate or asset any Lien which
(i) is imposed by Applicable Law, or (ii) in case of a security for Financial
Debt, is granted for debt fully reflected in the respective accounts of a
Material Messer Company or (iii) in case of a servitude, easement or similar
limitation, does not materially negatively affect the use of the relevant asset
whereby negative effects already existing on or prior to the date hereof shall
never be regarded as relevant.

"PERSON" means any individual, partnership, joint venture, corporation, limited
liability company, trust, unincorporated organization, Governmental Entity or
other entity.

"SITE" means one or more pieces of land forming a single production or
distribution facility which is used by an Environmental Company as of the
Closing Date.

"SPECIAL FOREIGN COMPANIES" means Messer Group Inc..

"SPECIAL NON-CONTROLLED COVERED ENTITY" means the entities described as such in
EXHIBIT 4.17.

"SOLD COVERED ENTITY" means the entities described as such in EXHIBIT 4.17.

"SUBSIDIARY" means with respect to any Person, a corporation, partnership,
association or other business entity of which (i) if a corporation, a majority
of the total voting power of shares of stock entitled (without regard to the
occurrence of any contingency) to vote in the election of directors is at the
time owned or controlled, directly or indirectly, by that Person or one or more
of the other Subsidiaries of that Person or a com-

<Page>

                                      -8-


bination thereof, or (ii) if a partnership, association or other business
entity, a majority of the partnership or other similar ownership interest
thereof is at the time owned or controlled, directly or indirectly, by that
Person or one or more Subsidiaries of that Person or a combination thereof. For
purposes hereof, a Person or Persons shall be deemed to have a majority
ownership interest in a partnership, association or other business entity if
such Person or Persons control the managing board or general partner of such
partnership, association or other business entity.

"TAX" means any (i) German federal, state or local tax within the meaning of
Section 3 of the German Tax Act (ABGABENORDNUNG) and (ii) respective non-German
federal, state or local tax, including for (i) and (ii) any interest, penalties
or additions to Tax or additional amounts with respect to the foregoing.

"TAX AUTHORITY" means any German or non-German Governmental Entity, agency, body
or other regulatory authority that is responsible for the assessment,
administration or collection of any Taxes in any jurisdiction.

"TAX INDEMNIFICATION COMPANY" means any Messer Group Company which is either a
Consolidated Messer Company or a Material Messer Company and all Messer Group
Companies which have their registered seat in India.

"TAX RETURN" means any return, report or similar written statement required to
be filed with any Tax Authority in respect of any Tax.

"TOTAL COMPENSATION" means the Main MGG Purchase Price plus the Hoechst NewCo II
Receivable Purchase Price plus the China Purchase Price; PROVIDED that if the
China SPA will not be consummated, the Total Compensation shall not include the
China Purchase Price. For the purpose of clarity only, the specific elements of
Total Compensation consist of

      (i)   the Loan Notes (Section 2.7(a)(iii)) in an amount of EUR 281,000,000
      (ii)  the Deferred Purchase Price (Section 2.7(a)(ii)) in an amount of EUR
            153,000,000
      (iii) a cash payment at Closing (Section 2.7(a)(i)) in an amount of EUR
            34,000,000

            ((i) through (iii) the Main MGG Purchase Price in the amount of EUR
            468,000,000)

<Page>

                                      -9-


      (iv)  the Hoechst NewCo II Receivable Purchase Price (Section 2.7(b))in an
            amount of EUR 200,000,000

      (v)   the China Purchase Price (Section 2.5) in an amount of
            EUR 32,000,000.

      Accordingly, the Total Compensation amounts to EUR 700,000,000.

      2.    TRANSACTIONS FORMING THE BUSINESS COMBINATION

            2.1   CERTAIN DATES. The following dates shall have the following
                  meanings and shall occur as described:

                  "FINANCIAL DATE" shall be August 31, 2000 24:00 h at each
                  jurisdiction affected.

                  "CLOSING DATE" shall be the fifth (5th) Business Day following
                  the day at which the conditions set forth in Section 3.1 have
                  been fulfilled, but not earlier than twenty (20) Business Days
                  after the notarization of this Agreement, unless otherwise
                  mutually agreed by Hoechst and Investor NewCo in writing.

            2.2   MGG'S PRE-CLOSING RESTRUCTURING. Prior to the execution of
                  this Agreement, MGG has contributed and transferred the
                  Special Foreign Companies to the capital of Messer Foreign
                  Holding on the basis of a contribution agreement, a copy of
                  which is attached as EXHIBIT 2.2, (the "FOREIGN SUB
                  CONTRIBUTION AGREEMENT"). MGG shall use reasonable best
                  efforts to have the capital increase connected with the
                  Foreign Sub Contribution Agreement registered as soon as
                  possible with the commercial register (the "FOREIGN STEP-UP").
                  It is agreed that Hoechst shall have no other responsibility
                  in relation to the foregoing than give to the extent legally
                  possible respective instructions to the management of MGG. If
                  requested by Investor NewCo, MGG agrees to promptly procure a
                  transfer of the Special Foreign Companies from Messer Foreign
                  Holding to a newly created special purpose vehicle (including
                  a VORRATSGESELLSCHAFT). Hoechst and MIG agree to procure the
                  foregoing.

<Page>

                                      -10-


      2.3   [Intentionally left blank].

      2.4   CONTRIBUTION AND TRANSFER OF HOECHST-MGG SHARE TO HOECHST NEWCO II.
            Promptly after the execution of this Agreement, Hoechst shall
            procure a capital increase of Hoechst NewCo II in such an amount
            that the stated capital of Hoechst NewCo II will amount to EUR
            60,000 and shall subscribe for such new capital. Such capital
            increase shall not be effected by a cash payment of Hoechst but
            through a contribution in kind. For such purpose Hoechst shall
            contribute to Hoechst NewCo II the Hoechst MGG Share, and Hoechst
            NewCo II shall accept such contribution of the Hoechst-MGG Share by
            executing a contribution agreement substantially as attached as
            EXHIBIT 2.4 (the "HOECHST NEWCO II CONTRIBUTION AGREEMENT").  In
            addition to the new shares in Hoechst NewCo II, Hoechst shall
            receive as consideration for the contribution of the Hoechst-MGG
            Share an amount of EUR 200,000,000 which Hoechst NewCo II shall owe
            to Hoechst as a loan (the "HOECHST NEWCO II CONTRIBUTION
            RECEIVABLE").

            Hoechst and Hoechst NewCo II shall initiate all acts and make all
            declarations necessary to procure that Hoechst NewCo II's capital
            increase shall be registered in the commercial register of Hoechst
            NewCo II as soon as possible, but not prior to the fulfillment of
            the closing condition set forth in Section 3.1(c).

      2.5   SALE OF CHINA INTERESTS(a) At the Closing Date MGG shall execute and
            Hoechst shall execute and procure that ACIC executes, an agreement
            substantially as attached in EXHIBIT 2.5 hereto (the "CHINA SALE AND
            PURCHASE AGREEMENT" or "CHINA SPA") regarding the direct or indirect
            sale and transfer to MGG or the MGG Designee (as defined in the
            China SPA) of the ACIC Gas Interests (as defined in the China SPA)
            and certain other assets of ACIC (the MCCO Assets as defined in the
            China Sale and Purchase Agreement). The purchase price to be set
            forth in the China Sale and Purchase Agreement shall amount to an
            aggregate of

                                 EUR 32 million
                          (the "CHINA PURCHASE PRICE")

<Page>

                                      -11-


                plus V.A.T., if applicable, at the applicable rate

      (b)   From the date of the China Sale and Purchase Agreement until the
            China Closing Date (as defined in the China Sale and Purchase
            Agreement) the China Purchase Price shall accrue interest at an
            annual rate of 200 basis points above three months EURIBOR,
            compounded annually and to be calculated on the basis of 360 days.
            At the Closing, Investor NewCo shall deliver to Hoechst an
            unconditional guarantee in the amount of the China Purchase Price,
            expiring not before the China Drop Date (as defined in the China
            Sale and Purchase Agreement) and issued by Investor NewCo (the
            "CHINA PURCHASE PRICE GUARANTEE").

      (c)   Those Exhibits to the China Sale and Purchase Agreement which have
            not yet been attached to the draft agreement attached as EXHIBIT 2.5
            hereto shall be agreed upon by the Parties prior to the Closing Date
            in good faith and in accordance with the facts and principles
            relevant to them under the China SPA.

      (d)   Prior to the Closing Date, the Parties shall (i) agree in good faith
            on whether and to what extent any provisions of the draft China Sale
            and Purchase Agreement attached as EXHIBIT 2.5 hereto shall, without
            being changed in substance, be moved prior to the execution of the
            China Sale and Purchase Agreement from the China Sale and Purchase
            Agreement into this Agreement or an Annex hereto and (ii) shall, if
            so agreed, amend this Agreement accordingly.

      (e)   In the case of a conflict between the China Sale and Purchase
            Agreement and this Agreement with respect to any issue dealt with in
            the China Sale and Purchase Agreement, the provisions of the China
            Sale and Purchase Agreement shall prevail.

      (f)   With respect to the allocation and charging of any fees for
            Management Services (as defined in the China SPA) Hoechst shall
            procure that in the period from the date of this Agreement until
            execution of the China SPA, ACIC will allocate and charge such fees
            only in accordance with past practice and with the principles and
            subject to the provisions set forth in the China SPA, PROVIDED, for
            the avoidance of doubt, that

<Page>

                                      -12-


            (i)   MGG shall pay

                  (A)   the undisputed amount of the Funding Deficit (as defined
                        in the China SPA) of RMB 22,8 million (EUR 3,1 million)
                        on or before January 31, 2001,

                  (B)   the remainder of the Funding Deficit as soon as the
                        verification procedure provided for in the ninth whereas
                        clause to the China SPA has resulted in an agreement as
                        to the amount of such remainder,

                  which payments shall both account for the cap set forth in
                  Section 7.2. of the China SPA and

           (ii)   without limiting any payment obligations of MGG under this
                  Agreement, until execution of the China SPA, ACIC shall not
                  call any amount under any guarantee or other enforceable
                  instrument issued with respect to the payment of fees and
                  expenses for Management Services by MGG or any Affiliate of
                  MGG in respect of such amounts, including, but not limited to,
                  the undertakings of MGG under the letters of MGG to ACIC dated
                  January 14 and September 13, 2000.

      2.6   SHARE LOAN OF HOECHST NEWCO II SHARES TO HOECHST NEWCO III. At the
            Closing Date, Hoechst and Hoechst NewCo III shall enter into a share
            loan agreement in form and substance as attached as EXHIBIT 2.6 (the
            "SHARE LOAN AGREEMENT") under which Hoechst lends to Hoechst NewCo
            III the Hoechst NewCo II Shares. The Share Loan Agreement shall
            provide for a maturity on January 12, 2002 and shall also provide
            that in case of an Aventis Credit Drop and a subsequent extension of
            the Call Option Period pursuant to Section 2.8., the maturity date
            shall be shifted to the date following the calendar day of the
            expiration of the extended Call Option Period.

      2.7   SALE OF HOECHST NEWCO II SHARES AND HOECHST NEWCO II CONTRIBUTION
            RECEIVABLE TO INVESTOR NEWCO.

<Page>

                                      -13-


            (a)   At the Closing Date after execution of the Share Loan
                  Agreement, Hoechst NewCo III shall sell and transfer to
                  Investor NewCo the Hoechst NewCo II Shares, and Investor NewCo
                  shall purchase and accept such transfer of the Hoechst NewCo
                  II Shares by executing a purchase and assignment agreement in
                  form and substance as attached as EXHIBIT 2.7(a)(1) (the
                  "HOECHST NEWCO II SHARE PURCHASE AGREEMENT I"). The purchase
                  price to be included in the Hoechst NewCo II Share Purchase
                  Agreement I as owed by Investor NewCo as consideration for the
                  sale and transfer of the Hoechst NewCo II Shares amounts to

                                         EUR 468,000,000

                                (the "MAIN MGG PURCHASE PRICE");

                  of which

                  (i)   EUR 34,000,000 are payable at the Closing Date;

                  (ii)  EUR 153,000,000 plus interest (the "DEFERRED PURCHASE
                        PRICE") are payable as set forth in Section 2.7(d) and

                  (iii) EUR 281,000,000 are not payable in cash but shall be
                        payable by way of delivery of one or two loan notes
                        (collectively the "LOAN NOTES") issued by Investor
                        NewCo and sellable at their aggregate face amount of EUR
                        281,000,000 at the Closing or immediately after the
                        Closing. Such Loan Notes are to be delivered at the
                        Closing Date, in a form reasonably satisfactory to
                        Hoechst.

                  It is acknowledged by the parties that the amount for the Main
                  MGG Purchase Price takes into account the amount of the
                  purchase price for the Hoechst NewCo II Contribution
                  Receivable.

            (b)   At the Closing Date Hoechst shall sell and transfer to
                  Investor NewCo the Hoechst NewCo II Contribution Receivable
                  for a purchase price equal to its face value (i.e., EUR
                  200,000,000, the "HOECHST NEWCO II RECEIVABLE PURCHASE PRICE")
                  pursuant to an assignment contract substantially as set forth
                  in EXHIBIT 2.7(b).

<Page>

                                      -14-


            (c)   The transfer of the Hoechst NewCo II Shares from Hoechst NewCo
                  III to Investor NewCo shall be subject to the condition
                  precedent of the receipt of (i) the Main MGG Purchase Price by
                  Hoechst NewCo III to the extent due and payable at the Closing
                  Date and in form of cash and notes as set forth in Section
                  2.7(a) and (ii) the Hoechst NewCo II Receivable Purchase Price
                  by Hoechst.

            (d)   The Deferred Purchase Price in the amount of EUR 153,000,000
                  is due and payable on the third anniversary of the Closing
                  Date, including interest thereon at a rate of 350 basis points
                  above three months EURIBOR, compounded annually and to be
                  calculated on the basis of 360 days, which interest shall be
                  paid together with the payment of the principal amount.
                  However, the Deferred Purchase Price shall become immediately
                  due and payable upon (A) a sale and transfer, including by
                  merger, of an equity interest of ACP or the GS Funds in
                  Investor NewCo which would result in ACP and the GS Funds
                  together no longer holding the majority of the votes and
                  equity interests in Investor NewCo (without taking into
                  account any shares, options, warrants or convertible
                  securities issued to management or to providers of debt
                  financing), (B) a sale of all or substantially all assets of
                  Investor NewCo, (C) any Insolvency of Investor NewCo or (D) a
                  liquidation of Investor NewCo.

                  Investor NewCo hereby covenants that until the date the
                  Deferred Purchase Price plus interest shall have been paid in
                  full

                  (i) Investor NewCo shall not make any dividend distributions
                  and not repay any shareholder loans and shall not grant any
                  loans except to its Subsidiaries or to the members of the
                  management of such Subsidiaries or as contemplated by the
                  Singapore Separation Agreement; PROVIDED that in connection
                  with a contribution of MIG's newly created interest in Hoechst
                  NewCo II to Investor NewCo, Investor NewCo may pay as
                  consideration to MIG one or more cash amounts promptly or at a
                  later date (including any loans created in connection
                  therewith and including any repay-
<Page>

                                      -15-


                  ments of such loans) as reflected in EXHIBIT 4.12(h).

                  (ii) Investor NewCo shall not make any payments under the Loan
                  Notes and hereby confirms that such Loan Notes will be junior
                  to the obligation to pay the Deferred Purchase Price such that
                  such Loan Notes shall not be paid before Hoechst NewCo III
                  shall have received the full amount of the Deferred Purchase
                  Price.

                  Investor NewCo shall be entitled to prepay the Deferred
                  Purchase Price at any time without any penalty.


      2.8   CALL OPTION, FOR HOECHST NEWCO II OPTION SHARES. Investor NewCo
            herewith grants to Hoechst NewCo III the option (the "CALL OPTION")
            to require Investor NewCo to sell and transfer to Hoechst NewCo III
            the Hoechst NewCo II Option Shares with economic and legal effect on
            January 11, 2002. Hoechst NewCo III may exercise the Call Option
            from January 01, 2002 until January 11, 2002, or any time before
            January 11, 2002 in case of an Insolvency of Investor NewCo, Hoechst
            NewCo II or MGG (the "CALL OPTION PERIOD"), PROVIDED that in case of
            an Aventis Credit Drop the Call Option Period shall upon request of
            Aventis be extended; PROVIDED that such request shall not cause the
            Call Option Period to terminate at a date later than ten (10) weeks
            after the Aventis Credit Drop has occurred. In case of such an
            extension of the Call-Option Period beyond January 11, 2002, all
            other periods relating to the Call Option, Counter-Call Option or
            Counter-Put Option shall be extended accordingly, including any
            payments dates for purchase prices due thereunder and also including
            the date upon which the sale and transfer of the Hoechst NewCo II
            Option Shares to Hoechst NewCo III hereunder shall become
            economically and legally effective.

            For the purpose of executing the Call Option, Investor NewCo hereby
            irrevocably offers to Hoechst NewCo III to sell and transfer the
            Hoechst NewCo II Option Shares within the Call Option Period as set
            forth in the share purchase agreement attached hereto as EXHIBIT
            2.8(a) (the "HOECHST NEWCO II SHARE PURCHASE AGREEMENT II"), subject
            to any necessary antitrust approvals, for a purchase price equal to
            the Main MGG Purchase Price (the "CALL OPTION PRICE"). Upon due
            exercise of the Call

<Page>

                                      -16-


            Option by Hoechst NewCo III, the Call Option Price becomes due and
            payable on January 31, 2002, subject to any necessary antitrust
            approvals.

            Hoechst NewCo III may exercise the Call Option by accepting the
            offer of Investor NewCo in writing within the Call Option Period, to
            be sent to Investor NewCo any time prior to the expiration of the
            Call Option Period, in which case, and only then, the Hoechst NewCo
            II Share Purchase Agreement II shall become effective without any
            further acts being required, PROVIDED, however, that the Call Option
            can only be exercised (i) upon delivery of a certificate to Investor
            NewCo set forth in EXHIBIT 2.8(b) and duly executed by the
            management of Hoechst, Hoechst NewCo II and Hoechst NewCo III, and
            (ii) subject to the following paragraph, the credit rating of
            Aventis has not dropped below BBB of Standard & Poor's credit rating
            as published by Standard & Poor's on or before December 1, 2001 (an
            "AVENTIS CREDIT DROP"). For purposes of delivery of such certificate
            Investor NewCo hereby irrevocably authorizes each of Dr. Christof
            Jackle of Hengeler Muller Weitzel Wirtz, Bockenheimer Landstrasse
            53, Frankfurt and Dr. Walter Henle of Baker & McKenzie,
            Theatinerstr. 23, 80333 Munchen, separately as agents of process
            to receive and confirm the receipt of the notice of Hoechst NewCo
            III to exercise the Call Option and such certificate.

            In case of an Aventis Credit Drop the Call Option can only be
            exercised by Hoechst NewCo III if Aventis shall have delivered to
            Investor NewCo an unconditional bank guarantee (UNBEDINGTE
            SELBSTSCHULDNERISCHE BANKGARANTIE) of an internationally recognized
            bank securing the rights of Investor NewCo under Section 4.14 in an
            amount of the Minimum Damage Amount. If so requested by Hoechst,
            Hoechst and Investor NewCo will promptly after an Aventis Credit
            Drop negotiate in good faith on an instrument replacing such bank
            guarantee as suggested by Hoechst with a view to provide comparable
            protection for Investor NewCo.

      2.9   COUNTER-CALL OPTION FOR HOECHST NEWCO III SHARES. Hoechst herewith
            grants to Investor NewCo the option (the "COUNTER-CALL OPTION") to
            require Hoechst to sell and transfer to Investor NewCo the Hoechst

<Page>

                                      -17-


            NewCo III Shares with economic and legal effect as of the day of the
            exercise of the Counter-Call Option. Investor NewCo may exercise the
            Counter-Call Option under the condition that Hoechst NewCo III has
            exercised the Call Option. Subject to the foregoing sentence
            Investor NewCo can exercise the Counter-Call Option between
            January 16, 2002 and January 23, 2002 (the "COUNTER-CALL OPTION
            PERIOD"). For the purpose of executing the Counter-Call Option
            Hoechst hereby irrevocably offers to Investor NewCo to sell and
            transfer the Hoechst NewCo III Shares within the Counter-Call Option
            Period under the terms and conditions set forth in the share
            purchase and assignment agreement attached hereto as EXHIBIT 2.9(a)
            (the "HOECHST NEWCO III SHARE PURCHASE COUNTER-CALL AGREEMENT"),
            subject to any necessary EU and US antitrust approvals, for a
            purchase price of EUR 25,000 (the "COUNTER OPTION PRICE"). Upon due
            exercise of the Counter-Call Option by Investor NewCo, the Counter
            Option Price becomes due and payable on January 31, 2002, subject
            to any necessary EU and US antitrust approvals. Investor NewCo may
            exercise the Counter-Call Option by accepting the foregoing offer
            of Hoechst by notarized (German or Basel notary) acceptance deed
            (for example in the form substantially as also set forth in
            EXHIBIT 2.9(b)) within the Counter-Call Option Period, in which
            case, and only then, the Hoechst NewCo III Share Purchase
            Counter-Call Agreement shall become effective, without any further
            acts being required.

            The Counter-Call Option (as well as the Counter-Put Option agreed in
            Section 2.10 hereinafter) shall be deemed to be duly and timely
            exercised if the acceptance deed has been notarized during the
            Counter-Call Option period irrespective of receipt of such deed by
            Hoechst or Investor NewCo, as the case may be. The then officiating
            notary shall be instructed to promptly deliver a notarized copy of
            the respective acceptance deed to the respective other party.

            For the avoidance of doubt: In case Hoechst NewCo III exercises the
            Call Option, Hoechst NewCo III shall subsequently transfer the
            Hoechst NewCo II Option Shares to Hoechst as part of its obligations
            under the Share Loan Agreement and Hoechst will have sold and
            transferred the Hoechst NewCo II Option Shares to a Subsidiary of
            Dresdner Bank (the

<Page>

                                      -18-


            "BANK SUB") on the basis of the Forward Sales Agreement I, and the
            Bank Sub will have transferred the Hoechst NewCo II Option Shares to
            Hoechst NewCo III on the basis of the Forward Sales Agreement II, so
            that Hoechst NewCo III is during the Counter-Call Option Period and
            the Counter-Put Option Period the sole owner of the Hoechst NewCo II
            Option Shares without any remaining obligations under the Share Loan
            Agreement or otherwise vis-a-vis Aventis or its Affiliates.

            The Counter-Call Option Period shall be shifted if, due to an
            Aventis Credit Drop, the Call Option Period is extended pursuant to
            Section 2.8. In that case, the Counter-Call Option Period shall
            commence 5 days after the expiration of the extended Call-Option
            Period and shall expire 12 days after the expiration of the extended
            Call Option Period. In such event the Counter-Call Option Price
            shall become due and payable twenty days after the expiration of the
            extended Call Option Period.

      2.10  COUNTER-PUT OPTION FOR HOECHST NEWCO III SHARES. Investor NewCo
            herewith grants to Hoechst the option (the "COUNTER-PUT OPTION") to
            require Investor NewCo to buy and accept transfer from Hoechst of
            the Hoechst NewCo III Shares with economic and legal effect as of
            the day of the exercise of the Counter-Put Option. Hoechst may
            exercise the Counter-Put Option under the condition that Hoechst
            NewCo III has exercised the Call Option and Investor NewCo has not
            exercised the Counter-Call Option. Subject to the foregoing sentence
            Hoechst can exercise the Counter-Put Option between January 24, 2002
            and January 31, 2002 (the "COUNTER-PUT OPTION PERIOD"). For the
            purpose of executing the Counter-Put Option Investor NewCo hereby
            irrevocably offers to Hoechst to buy and accept transfer of the
            Hoechst NewCo III Shares within the Counter-Put Option Period under
            the terms and conditions set forth in the share purchase and
            assignment agreement set forth in EXHIBIT 2.10(a) (the "HOECHST
            NEWCO III SHARE PURCHASE COUNTER-PUT AGREEMENT"), subject to any
            necessary EU and US antitrust approvals, for a purchase price equal
            to the Counter Option Price (the "COUNTER-PUT OPTION PRICE"). Upon
            due exercise of the Counter-Put Option by Hoechst, the Counter
            Option Price becomes due and payable on January 31, 2002, subject to
            any necessary EU and US antitrust approvals. Hoechst may exercise
            the Counter-Put Option by accepting the forego-
<Page>

                                      -19-


            ing offer of Investor NewCo by notarized (German or Basel notary)
            acceptance deed (for example in the form substantially as set forth
            in EXHIBIT 2.10(b)) within the Counter-Put Option Period, in which
            case, and only then, the Hoechst NewCo III Share Purchase
            Counter-Put Agreement shall become effective without any further
            acts being required.

            The Counter-Put Option Period shall be shifted if, due to an Aventis
            Credit Drop, the Call Option Period is extended pursuant to
            Section 2.8, and the Counter-Call option Period is consequently
            shifted pursuant to Section 2.9. In that case, the Counter-Put
            Option Period shall commence 13 days after the expiration of the
            extended Call Option Period and shall expire 20 days after the
            expiration of the extended Call Option Period, upon which date the
            Counter-Put Option Price shall become due and payable.

      2.11  SUMMARY OF STEPS. The steps described in Sections 2.2 through 2.10
            may be summarized for illustration purposes as follows:

<Table>
<Caption>
- --------------------------------------------------------------------------------------------------
                                                                                  SECTION IN
  NO.    TIME                             ACTION                                   AGREEMENT
- --------------------------------------------------------------------------------------------------
                                                                             
   1     In 2000 and before Closing       Step-up of Special Foreign                  2.2
         Date                             Companies by contribution to
                                          Messer Foreign Holding.
- --------------------------------------------------------------------------------------------------
   2     Before Closing Date              Contribution by Hoechst of                  2.4
                                          Hoechst-MGG Share to Hoechst
                                          NewCo II.
- --------------------------------------------------------------------------------------------------
   3     Closing Date                     Execution of China SPA; delivery            2.5
                                          of China Purchase Price
                                          Guarantee.
- --------------------------------------------------------------------------------------------------
   4     Closing Date                     Share loan by Hoechst of Hoechst            2.6
                                          NewCo II Shares to Hoechst
                                          NewCo III.
- --------------------------------------------------------------------------------------------------
   5     Closing Date                     Sale and transfer by Hoechst                2.7
                                          NewCo III of Hoechst NewCo II
                                          Shares and by Hoechst of claims
                                          from Hoechst NewCo II
                                          Contribution Receivable to
                                          Investor NewCo; partial
                                          "payment" of Main MGG
                                          Purchase Price; and payment of
                                          Hoechst NewCo II Receivable
                                          Purchase Price.
- --------------------------------------------------------------------------------------------------

<Page>

                                      -20-


<Caption>
- --------------------------------------------------------------------------------------------------
                                                                                  SECTION IN
  NO.    TIME                             ACTION                                   AGREEMENT
- --------------------------------------------------------------------------------------------------
                                                                           
   6     Closing Date                     Execution of Forward Sale                 3.1(b)
                                          Agreements I and II.
- --------------------------------------------------------------------------------------------------
   7     Closing Date                     Payments by Hoechst for Covered           4.17(m)
                                          Guarantees.
- --------------------------------------------------------------------------------------------------
   8     Until January 11, 2002 (subject  Possible exercise by Hoechst                2.8
         to extension of Call Option      NewCo III of Call Option; then
         Period in case of Aventis        transfer of Hoechst NewCo II
         Credit Drop)                     Option Shares by Investor NewCo
                                          to Hoechst NewCo III.
- --------------------------------------------------------------------------------------------------
   9     January 12, 2002 (subject to     Transfer of Hoechst NewCo II                2.6
         extension of Call Option         Option Shares from Hoechst
         Period to be postponed to first  NewCo III to Hoechst pursuant to
         day after expiration of          Share Loan Agreement.
         extended Call Option Period)
- --------------------------------------------------------------------------------------------------
   10    January 13, 2002 (subject to     Transfer of Hoechst NewCo II                2.9
         extension of Call Option         Option Shares to Bank Sub and to
         Period to be postponed to        Hoechst NewCo III pursuant to
         second day after expiration of   Forward Sale Agreement I and II
         extended Call option Period)
- --------------------------------------------------------------------------------------------------
   11    Between January 16, 2002 and     Possible exercise by Investor               2.9
         January 23, 2002 (subject to     NewCo of Counter-Call Option
         extension of Call Option         against Hoechst; then transfer of
         Period to commence 5 days        Hoechst NewCo III Shares from
         after the expiration of the      Hoechst to Investor NewCo.
         extended Call-Option Period
         to expire 12 days after the
         expiration of the extended Call
         Option Period)
- --------------------------------------------------------------------------------------------------
   12    Between January 24, 2002 and     Possible exercise by Hoechst of             2.10
         January 31, 2002 (subject to     Counter-Put Option against
         extension of Call Option         Investor NewCo; then transfer of
         Period to commence 13 days       Hoechst NewCo III Shares from
         after the expiration of the      Hoechst to Investor NewCo.
         extended Call Option Period
         and to expire 20 days after the
         expiration of the extended Call
         Option Period)
- --------------------------------------------------------------------------------------------------
   13    24 months after Closing          Final date for completion of                2.5
                                          China SPA; payment of China
                                          Purchase Price at China Closing
                                          Date.
- --------------------------------------------------------------------------------------------------
   14    No later than the third          Payment of Deferred Purchase               2.7(d)
         anniversary of the Closing       Price
         Date
- --------------------------------------------------------------------------------------------------
</Table>

      2.12  CLOSING TRANSACTIONS. The closing of the transactions contemplated
            by this Agreement to be consummated shall take place on the
            respective dates at such time and place as set forth in the
            Combination Documents

<Page>


                                      -21-

            or as the parties hereto designate in writing. The "CLOSING TIME"
            shall be deemed to occur in each jurisdiction affected at 00:01
            a.m. (local time) on the respective date.

            Following the Closing, each party shall execute any additional
            documents or perform such acts or measures as reasonably required by
            any other party which are necessary or reasonably advisable to fully
            consummate the relevant transactions to be consummated hereunder;
            PROVIDED that no party is obliged to assume any particular costs not
            otherwise contemplated in this Agreement as a result of the
            foregoing.

3.    CONDITIONS TO CLOSING

      3.1   CONDITIONS TO THE PARTIES' OBLIGATIONS. The obligation of the
            parties hereto to consummate the transactions contemplated by this
            Agreement to be performed on the Closing Date is subject to the
            satisfaction of the following conditions (each of which -- except
            for the condition stated in (c) below -- may be partially or
            entirely waived by all the parties hereto in their sole discretion
            with respect to the respective party's obligation to close) as of
            the Closing Date; PROVIDED, however, that the closing conditions set
            forth (i) in para. (b), (f), (g), (h), (j), (l) (m), (n) and (o) of
            this Section 3.1 shall only be for the benefit of Investor NewCo,
            and (ii) in para. (k) and (p) of this Section 3.1 shall only be for
            the benefit of Hoechst.

            (a)   The relevant parties shall have executed each of the Ancillary
                  Agreements.

            (b)   Each of Hoechst and Hoechst NewCo III shall have executed
                  forward sale agreements with Bank Sub (or any other
                  internationally recognized bank) substantially as attached in
                  EXHIBIT 3.1(b)(i) in case of Hoechst (the "FORWARD SALE
                  AGREEMENT I"), and in EXHIBIT 3.1(b)(ii) in case of Hoechst
                  NewCo III (the "FORWARD SALE AGREEMENT II"), as a result of
                  which Hoechst NewCo III will receive the Hoechst NewCo II
                  Shares two days after the expiration of the Call Option
                  Period. Such agreements

<Page>

                                      -22-


                  shall be in full force and effect and not terminated as of the
                  Closing Date.

            (c)   The transactions contemplated by this Agreement can be
                  lawfully consummated under Regulation (EU) 4064/89 as amended
                  (the "EU REGULATION"), and/or EU member state merger control
                  laws in Germany, Austria and Ireland and the waiting period
                  applicable to such transactions under the Hart-Scott-Rodino
                  Antitrust Improvements Act of 1976 as amended (the "HSR ACT"),
                  if applicable, shall have expired or been terminated (such
                  approvals under the EU Regulation and applicable EU members
                  states merger control laws and such expiration or termination
                  under the HSR Act being collectively referred to herein as the
                  "ANTITRUST APPROVALS").

            (d)   Each of the parties shall have in all material respects duly
                  performed and complied with all of its covenants and
                  obligations in this Agreement required to be so performed or
                  complied with by the party concerned prior to the Closing
                  Date.

            (e)   The capital increase of Hoechst NewCo II resulting from the
                  execution of the Hoechst NewCo II Contribution Agreement shall
                  have been registered in the commercial register of Hoechst
                  NewCo II.

            (f)   Between the date hereof and the Closing Date, there have not
                  been one or more occurrences or events out of the usual course
                  specifically relating to a Messer Group Company, each material
                  by effect, that have caused or would reasonably be expected to
                  cause, individually or in the aggregate, an adverse change on
                  or with respect to the Messer Group and/or the business of the
                  Messer Group, in each case taking the Messer Group as a whole,
                  which adverse change would result in a decrease in the Equity
                  Value of the Messer Group during the period between the date
                  hereof and the Closing Date by at least EUR 125,000,000;
                  PROVIDED that a change, development, occurrence or condition
                  resulting from general economic conditions or in the business
                  sector in which the Messer Group operates shall not be taken
                  into
<Page>

                                      -23-


                  account. For purposes of the foregoing, the "EQUITY VALUE" as
                  of any particular date means the fair market value of all
                  shares of MGG. It is acknowledged and agreed that for purposes
                  of the foregoing the Equity Value of the Messer Group as of
                  the date hereof amounts to EUR 1,002,000,000 (i.e., 150% of
                  the Total Compensation without the China Purchase Price). For
                  purposes of comparing Equity Value as of the date hereof to
                  Equity Value as of any later date, Equity Value as of both
                  such dates shall take into account, and assume, the full
                  implementation of the arrangements and transactions
                  contemplated by the Combination Documents.

            (g)   The representations and warranties of Hoechst contained in
                  Section 5 of this Agreement shall be true and correct in all
                  respects as of the Closing Date or as of any other date, if
                  specifically made as of such other date; PROVIDED that the
                  foregoing condition shall only be deemed not satisfied if the
                  aggregate Losses arising out of or resulting from, or which
                  could reasonably be expected to arise of or result from, all
                  such representations and warranties which turn out not to be
                  true and correct as of the dates they were given -- determined
                  without deducting or otherwise taking into account any
                  applicable De Minimis Amount or the Threshold -- exceed EUR 50
                  million.

            (h)   No Insolvency, as defined hereinafter, shall have occurred
                  with respect to any of the Messer Group Companies listed in
                  EXHIBIT 3.1(h) and none of the Messer Group Companies listed
                  in EXHIBIT 3.1(h) shall have become insolvent, as defined
                  hereinafter. "INSOLVENCY" means bankruptcy or composition
                  proceedings have been applied for or such application has been
                  rejected due to lack of assets or circumstances exist which
                  would compel such proceedings. "INSOLVENT" means in respect of
                  the respective company that (i) it is unable to pay debts as
                  they fall due or it suspends making payments on its debts,
                  (ii) a moratorium is declared or otherwise occurs in respect
                  of any of/all or any class of its indebtedness, (iii) the
                  commencement of formal negotiations with creditors with the
                  objective to generally reschedule indebt-
<Page>

                                      -24-


                  edness by reason of actual or anticipated financial
                  difficulties, (iv) its shareholders take any steps or legal
                  proceedings are commenced in relation to its winding-up,
                  liquidation, dissolution or reorganization or any analogous
                  procedure in any jurisdiction if such proceedings have not
                  been discharged on or before the Closing Date, or (v) it or
                  its shareholders take any steps or legal proceedings are
                  commenced in relation to the appointment of a liquidator,
                  receiver, administrator, administrative receiver, compulsory
                  manager or other similar officer in respect of the relevant
                  company or any of its material assets if such steps or
                  proceedings have not been terminated or discharged on or
                  before the Closing Date.

            (i)   [intentionally left blank]

            (j)   Hoechst shall have delivered to Investor NewCo an
                  unconditional guarantee of Aventis, substantially as set forth
                  in EXHIBIT 3.1(i), accompanied by a legal opinion rendered by
                  Shearman & Sterling (Paris), in form and substance reasonably
                  acceptable to Investor NewCo.

            (k)   Hoechst shall have received the China Purchase Price
                  Guarantee.

            (l)   Hoechst, Aventis, MIG, MGG, Investor NewCo and certain special
                  purpose vehicles shall have executed a Formation, Funding and
                  Shareholders' Agreement relating to Singapore Syngas Pte. Ltd.
                  and Messer Singapore Pte. Ltd. (the "SINGAPORE SEPARATION
                  AGREEMENT") substantially as set forth in EXHIBIT 3.1(l).

            (m)   Investor NewCo shall not have reasonably determined that the
                  representations and warranties contained in Section 5.20 are
                  not true and correct in any respect, either when made or as of
                  the Closing Date.

            (n)   No event or development shall have occurred, and no
                  information shall have come to the attention of Investor
                  NewCo, which causes Investor NewCo to reasonably conclude that
                  there is a reasonable likelihood that the investigation
                  referred to in EXHIBIT D will result in the indictment or
                  criminal or civil prosecution
<Page>

                                      -25-


                  (including the entry of any guilty plea, consent decree or
                  similar resolution) of any Messer Group Company or any current
                  officer or director of any Messer Group Company.

            (o)   As of the Closing Date, no Messer Group Company, Aventis China
                  Company or ACIC (to the extent related to the China
                  Interests) shall be involved, directly or indirectly, in any
                  activity (other than the business activities of certain Messer
                  Group Companies in Cuba) which, following consummation of the
                  transactions contemplated by the Combination Documents, would
                  constitute a violation on the part of any Material Messer
                  Company, Investor NewCo or any shareholder of Investor NewCo,
                  of the laws of the United States by virtue of Investor NewCo
                  being controlled by an entity that is organized in and subject
                  to the jurisdiction of the United States and, if applicable,
                  Investor NewCo or its appropriate Affiliate having failed to
                  obtain any required authorization or license so that they can
                  lawfully consummate the transactions contemplated herein
                  (other than the China SPA) under the laws of the United
                  States. It is understood that Investor NewCo and MGG will not
                  be obliged to consummate the China SPA if at the China Closing
                  Date this condition is not satisfied with respect to the
                  transactions contemplated by the China SPA.

            (p)   Hoechst shall have received one or more commitments of one or
                  more internationally recognized financial institutions, as
                  referred to and further described in Section 4.18, reasonably
                  satisfactory to Hoechst.

      3.2   NON-FULFILLMENT OF CLOSING CONDITIONS. If the non-fulfillment of any
            closing condition results from, or may be attributable to, a breach
            by any party of any representation and warranty or covenant or
            obligations hereunder, then such breaching party shall not be
            entitled to the benefit of this condition in respect of such consent
            or approval. No party is entitled to refuse to consummate or to
            delay the consummation of any of the transactions contemplated
            hereunder for any reason other than the non-satisfaction of any
            conditions made for its benefit, all as set forth in Section 3.1;
            PROVIDED that Hoechst's and Hoechst NewCoIII's respective

<Page>

                                      -26-


            obligations are subject to the receipt of the portion of the Total
            Compensation by Hoechst and Hoechst NewCo III which is due at the
            Closing Date, respectively.

      3.3   EXEMPTION OF CERTAIN TRANSACTION. If and to the extent the
            transactions to be performed on the Closing Date cannot be lawfully
            performed under the laws and regulations of specific countries
            outside the EU and the US due to merger control proceedings or other
            regulatory requirements despite the fulfillment of all closing
            conditions set forth in Section 3.1, the transactions to be
            performed on the Closing Date shall be performed to the maximum
            extent legally possible. The parties shall in such case agree on all
            appropriate measures to comply with such requirements including
            "hold separate" arrangements so that the affected countries can be
            exempted for the time being (until consummation is permitted) from
            the consummation of the remaining transaction. Hoechst shall comply
            with all reasonable requests of Investor NewCo as to the form and
            structure of such hold separate structure or any alternative
            mechanism or arrangement if and to the extent it does not lead to
            additional costs or liabilities of Hoechst. If the respective
            requirements are not met ten (10) months after the Closing Date,
            each of Hoechst and Investor NewCo can request a sale of the Messer
            Group Companies concerned to a third party, which sale shall be
            effected to the extent legally permissible in accordance with the
            instructions reasonably given by Investor NewCo. The proceeds of
            such sales shall be transferred by the Seller to MGG or such other
            companies as instructed by Investor NewCo net of taxes and fees and
            other expenses made. The foregoing shall not delay the Closing Date
            and the obligation of Investor NewCo to pay or procure the payment
            of the portions of the Total Compensation which are due at the
            Closing Date.

      3.4   MAC DETERMINATION AND BREACH OF REP DETERMINATION. In the event that
            Investor NewCo determines that the condition set forth in
            Section 3.1(f) has not been satisfied, it shall promptly notify
            Hoechst of such determination and provide Hoechst with a written
            statement briefly setting forth the basis for such determination
            (a "MAC DETERMINATION NOTICE"). In the event that Hoechst disagrees
            with such determination, it shall within five (5) Business Days of
            its receipt of such notice and statement from In-

<Page>

                                      -27-


            vestor NewCo, notify Investor NewCo of such disagreement and provide
            Investor NewCo with a statement briefly setting forth the basis for
            such disagreement (a "MAC DISPUTE NOTICE"). Such disagreement shall
            then promptly be submitted for resolution to UBS Frankfurt
            Investment Banking. In the event UBS is not prepared to act
            hereunder including to comply with the timetable provided
            hereinafter, Hoechst may propose an alternative investment bank,
            reasonably acceptable to Investor NewCo, to replace UBS. Such
            investment bank firm shall be directed to make a determination
            within fifteen (15) Business Days after engagement of whether the
            condition set forth in Section 3.1(f) has been satisfied or not,
            and which determination (the "FINAL MAC DETERMINATION") shall be
            final and binding upon all parties. The condition set forth in
            Section 3.1(f) shall be deemed not to have been satisfied during
            the period between the delivery of a MAC Determination Notice and
            the delivery of the Final MAC Determination. Following the delivery
            of the Final MAC Determination, (a) if the condition set forth in
            Section 3.1(f) has been determined to have not been satisfied,
            this Agreement may be terminated pursuant to Section 10.1(a)
            subject to the terms and conditions of such provision, and (b) if
            the condition set forth in Section 3.1(f) has been determined to
            have been satisfied, it shall for all purposes be deemed to have
            become satisfied at the date the Closing Date would have otherwise
            occurred without regard to proceedings set forth in this Section
            3.4. In the event that following delivery of a MAC Determination
            Notice, Hoechst fails to deliver a MAC Dispute Notice within the
            time period specified above for that purpose, the condition set
            forth in Section 3.1(f) shall be deemed for all purposes to have
            not been satisfied, and either party may thereafter terminate this
            Agreement pursuant to Section 10.1(a), subject to the terms and
            conditions of such provision. Until the Final MAC Determination has
            been rendered neither party is permitted to terminate this
            Agreement according to Section 10.1(a). If Investor NewCo
            wrongfully alleged a non-satisfaction of the condition set forth in
            Section 3.1(f), the Closing Drop Date shall be postponed for a
            period of five (5) Business Days after the Final MAC Determination,
            if this is after the Closing Drop Date, and the parties shall close
            the transactions contemplated to be closed at the Closing Date
            before any such new Closing Drop Date if all other closing
            conditions are fulfilled as provided for in this Agreement; PROVIDED
            however that the Closing Drop Date shall not be postponed be-

<Page>

                                      -28-


            yond May 15, 2001. The cost of the investment bank shall be
            allocated by applying Section 91 German Civil Procedure Code
            accordingly. Advance payments to the investment bank firm shall be
            made by Hoechst and Investor NewCo in equal amounts.

            The foregoing shall apply, MUTATIS MUTANDIS, in case of a dispute
            between the parties whether the closing condition set forth in
            Section 3.1(g) is fulfilled or not; PROVIDED that the breach must
            be determined by a law firm to be nominated by the investment bank.

4.    COVENANTS

      4.1   GENERAL. Subject to the terms of this Agreement and without limiting
            the obligations of the parties specified otherwise in this
            Agreement, each party shall perform its obligations hereunder and
            shall exert best efforts to cause each of the conditions set forth
            in Section 3.1 to be satisfied as soon as possible; PROVIDED that
            except as specifically provided herein, Hoechst and MIG shall not be
            obliged to exert any efforts with respect to the business or
            financial situation of the MGG Group or with respect to
            Section 3.1(o), and further PROVIDED that the obligations of
            Investor NewCo with respect to the conditions in Sections 3.1(c)
            and 3.1(p) are limited to what is set forth in Sections 4.9 and
            10.2, respectively, except if expressly agreed otherwise. Hoechst
            shall procure that the Ancillary Agreements will be timely
            executed and use reasonable best efforts to have such agreements
            implemented by respective registrations at the respective commercial
            registers, if applicable, so that the Closing Date will not be
            delayed. No party shall have any right to delay or refuse the
            Closing for any reason other than the non-fulfillment of the
            conditions set forth in Section 3.1. In particular, in no event
            shall an improvement or deterioration of the Gas Business, nor
            events affecting the Gas Business be a reason to delay or refuse
            the Closing, all except otherwise provided for in Section 3.1(f),
            3.1(g) and 3.1(h) above. Furthermore, other than as provided in
            Section 3.1(c) and 3.1(o) any missing consent, approval or
            authorization of, permit from, or declaration, filing or
            registration with, any governmental or regulatory authority, or
            any other person or entity, required to be made or obtained for
            the consummation of this

<Page>

                                      -29-


            Agreement with respect to merger control legislation, foreign
            investment and foreign securities and exchange legislation shall not
            prevent or delay the Closing; Section 3.3 shall remain unaffected.
            Notwithstanding the foregoing, the parties shall cooperate in good
            faith to obtain all required approvals of governmental authorities
            and agencies as well as of third parties (including banks or
            co-shareholders in Messer Group Companies) to consummate the
            transaction contemplated hereunder.

      4.2   COVENANTS OF HOECHST AND MIG. Hoechst and MIG have given certain
            instructions to the management (GESCHAFTSFUHRUNG) of MGG by letter
            dated September 5, 2000; a copy of such letter is attached as
            EXHIBIT 4.2. Such instructions shall not be revoked or changed prior
            to the Closing Date. Hoechst has not granted and shall not grant any
            consent which might be required for any measures under such
            instruction without the prior written consent of Investor NewCo
            which shall not be unreasonably withheld or delayed. If and to the
            extent Hoechst believes that applicable law may be violated with
            respect to a specific matter requiring consent of Investor NewCo,
            the parties will discuss the matter in good faith with a view to
            come to mutually acceptable solutions.

            If Hoechst or MIG become aware that such instructions have not been
            or are not followed, they shall promptly inform Investor NewCo and
            shall undertake all reasonable steps to ensure that these
            instructions are being followed. For the avoidance of doubt: Nothing
            contained in this Section 4.2 shall be construed as an obligation of
            Hoechst or MIG to actively manage the Gas Business.

            Furthermore, it is agreed that Hoechst may unilaterally procure
            (without assuming a respective obligation) that MGG may undertake
            appropriate activities including the contribution of cash to avoid
            an Insolvency of the Messer Group Companies referred to in EXHIBIT
            3.1(h) or such companies becoming Insolvent.

      4.3   NEGATIVE COVENANTS OF HOECHST. Hoechst shall procure that MGG has
            not made and shall not make any dividend distributions after the
            Financial Date up to the Closing Date.

<Page>

                                      -30-


      4.4   HOECHST NOMINEES. Upon request by Investor NewCo Hoechst shall cause
            all members of the supervisory board of MGG and of the shareholders'
            committee of MGG and of the corporate bodies of the Messer Group
            Companies, if any, to resign as of the Closing Date if legally
            possible, otherwise with effect as soon as possible, all to the
            extent such persons are nominated by Hoechst and such persons are
            not employees of the Messer Group Companies.

      4.5   ACCESS AND INFORMATION. Hoechst and MIG shall procure to the extent
            legally permissible under applicable anti-trust laws that after the
            signing of this Agreement through the Closing Date, Investor NewCo
            and its representatives and advisors shall have regular access to
            the top management of MGG to receive updates of important new
            developments relating to the industrial gas market or the Gas
            Business which are reasonably expected to be of potential interest
            for Investor NewCo. In addition, such top management of MGG shall be
            instructed by Hoechst and MIG to grant further access to information
            of the Gas Business and the financial position of MGG and, as may
            be, assistance reasonably requested by Investor NewCo from time to
            time (to include all information financial or otherwise, and all
            (whether audited or unaudited) interim and/or final accounts
            required for a high yield or other financing including such top
            management's assistance with syndication, marketing and
            registrations at a stock exchange and the execution or release of
            any security documentation, subject to customary confidentiality
            obligations. It is understood that no access granted to Investor
            NewCo hereunder shall unduly interfere with the business operations
            of the Messer Group Companies.

      4.6   FINANCIAL STATEMENTS 2000. The parties are in agreement that the
            Financial Statements 2000 shall be prepared in accordance with
            Applicable Law and IAS including a reconciliation into U.S. GAAP for
            the consolidated financial statements and with Applicable Law for
            the individual financial statements on a basis consistent with the
            respective financial statements prepared for the 1999 financial
            year. The parties acknowledge that according to MGG management MGG
            has to write down the book value of certain Messer Group Companies
            as further described in EXHIBIT 4.6 with effect as of December 31,
            2000 (the "WRITE-DOWNS")

<Page>

                                      -31-


            and will use reasonable best efforts that such Write-Downs and the
            Foreign Sub Contribution Agreement as consummated pursuant to
            Section 2.2 will be sustained in connection with the preparation and
            audit of the Financial Statements 2000, as permitted by Applicable
            Law. Subject to the timing of the Closing Date, each party agrees to
            use its reasonable best efforts that such Write-Downs and the
            profits resulting from the Foreign Sub Contribution Agreement will
            be accepted as appropriate (not too high and not too low) by the
            Company's statutory auditor. The Parties agree that it was not
            required to make the Write-Downs earlier than in the Financial
            Statements 2000; therefore, the non-inclusion of the Write-Downs in
            the Financial Statements 1999 shall not constitute a violation of
            the representation in Section 5.5 and 5.6. Furthermore, any time
            prior to Closing Hoechst and MIG will keep Investor NewCo fully
            informed and will cooperate (including with respect to certain
            disclosures which may be reasonably requested in connection with the
            Financing), if reasonably required by Investor NewCo, in connection
            with the preparation and audit of the Financial Statements 2000, and
            any time after the Closing Investor NewCo and MIG will VICE VERSA
            keep Hoechst fully informed and cooperate with Hoechst. After the
            Closing Investor NewCo shall procure that the MGG Group delivers all
            financial documentation reasonably required by Aventis for its
            financial reporting and relating to the period up to the Closing
            Date.

      4.7   BUSINESS YEAR. Prior to Closing, Hoechst and MIG shall use
            reasonable best efforts to change the business year of MGG and
            Hoechst NewCo II (subject to Section 4.12(a)) if and as requested by
            Investor NewCo, to a term to be proposed by Investor NewCo.

      4.8   COVENANTS OF INVESTOR. Reference is made to the Equity Commitment
            Letters delivered by GS Capital Partners 2000, L.P. and certain
            affiliated funds (collectively the "GS FUNDS"), Allianz AG and
            Allianz Capital Partners GmbH as of the date of notarization of this
            Agreement, and copies of such letters are attached hereto for
            evidence purposes only as EXHIBIT 4.8. Investor NewCo hereby agrees
            to procure that the GS Funds will deliver a legal opinion as and
            when contemplated by the Equity

<Page>

                                      -32-


            Commitment Letter of the Fund which legal opinion shall be
            reasonably acceptable to Hoechst.

      4.9   ANTITRUST REGULATORY APPROVALS.

            (a)   Each party shall, without undue delay and as promptly as
                  practicable, but in any event within the required statutory
                  periods, and in order to facilitate prompt consummation of the
                  transactions contemplated hereby, file or cause to be filed
                  with any antitrust authorities any notifications required to
                  be filed under any laws or regulations that are applicable to
                  the transactions contemplated hereby (collectively, the
                  "ANTITRUST FILINGS"). Antitrust Filings that are not required
                  to be made personally by any of the parties shall be made by
                  Investor NewCo. The parties shall diligently cooperate to (i)
                  make the Antitrust Filings each at their respective costs
                  without undue delay, and (ii) respond to any requests for
                  additional information made by any applicable antitrust
                  authorities in a timely, complete and correct manner.

            (b)   Investor NewCo shall use every reasonable effort (including,
                  without limitation, payment of any required fees) to (i)
                  obtain promptly all consents of any applicable antitrust
                  authority necessary for the Closing (except for such consents
                  the failure of which to obtain would not prevent or materially
                  delay the Closing); (ii) avoid the issuance of, or to have
                  vacated or terminated, any decree, order, or judgment of any
                  anti-trust authority or court that would restrain, prevent or
                  delay the Closing, including without limitation defending
                  through litigation on the merits any claim asserted in any
                  court by any party; PROVIDED that neither party shall be
                  obligated to comply with any conditions imposed by any
                  antitrust authority except for the obligation to comply with
                  anything that relates to shareholder rights within Investor
                  NewCo (i.e., adjustments to the individual shareholder rights
                  of Allianz Capital Partners GmbH or the GS Funds).

            (c)   Each party shall promptly notify the other parties of any
                  material communication to that party from any applicable
                  antitrust

<Page>

                                      -33-


                  authority and consult with the other parties regarding any
                  proposed communication to an applicable antitrust authority.
                  Each party shall consult with the others regarding any meeting
                  with any applicable antitrust authority in respect of any
                  filings, investigation or other inquiry, and to the extent
                  appropriate give the other parties the opportunity to attend
                  and participate thereat. Subject to the joint defense
                  privilege, each of the parties will coordinate and cooperate
                  fully with the others in exchanging such information (either
                  directly or through counsel and except for business secrets)
                  and providing such assistance as the others may reasonably
                  request in connection with the foregoing and in seeking early
                  termination of any applicable waiting periods under the HSR
                  Act and the EU Regulation or in connection with other
                  consents. The parties and their respective representatives
                  shall exchange copies of correspondence, filings or
                  communications (or memoranda setting forth the substance
                  thereof) between such party or any of its representatives, on
                  the one hand, and any applicable antitrust authority or
                  members of its staff, on the other hand, with respect to this
                  Agreement and the transactions contemplated hereby.

            (d)   It is being understood and agreed that the respective
                  Antitrust Filings shall in any event include a full
                  description of the participation and partnership relation
                  regarding Investor NewCo and of the totality of transactions
                  agreed upon hereunder, including the transaction steps to be
                  implemented in 2002, and any antitrust approvals shall, to the
                  extent reasonably possible, include the possible transactions
                  in 2002 (i.e., Call Option and Counter-Call Option or
                  Counter-Put Option, Forward Sale Agreements I and II),with a
                  view to avoid, to the extent possible, any such further
                  antitrust approvals necessary to consummate such transactions
                  in 2002.

      4.10  PENSIONSKASSE IN GERMANY. With respect to the Pensionskasse of
            Hoechst, which is a multiple-employer pension plan, currently
            participating employees of the Messer Group (the "GERMAN
            PARTICIPATING EMPLOYEES") will be allowed in accordance with the
            statutes of the Pension-

<Page>


                                      -34-

            skasse to continue their participation in this plan. This applies as
            long as the German Participating Employees are employed by the
            Messer Group. Any future new hires of Messer Group Companies
            headquartered in Germany will also be allowed to join the
            Pensionskasse. In case Investor NewCo, after the Closing Date, wants
            to terminate such membership or offer participation in another
            pension plan to the German Participating Employees, it is understood
            that as at such termination date (i) all accrued pension benefits
            will be maintained by the Pensionskasse (AUSSERORDENTLICHE
            MITGLIEDSCHAFTEN) and (ii) no assets will be transferred into any
            successor pension plan. For the avoidance of doubt: The foregoing is
            subject to the provisions of the Pensionskasse, as amended from time
            to time, and in particular if the employment terminates before
            retirement, the regulations provided in the statutes of the
            Pensionskasse shall apply.

      4.11  [intentionally left blank]

      4.12  COVENANTS OF INVESTOR NEWCO. With a view to protect the economic
            interests of Hoechst and Hoechst NewCo III with respect to a
            potential exercise of the Call Option, Counter-Call Option or
            Counter-Put Option, Investor NewCo hereby agrees as follows and to
            procure as follows for the time between the Closing Date and the
            exercise of the Call Option and, if still under its control, until
            the exercise of the Counter-Call Option or the Counter-Put Option or
            until the date the Call Option can no longer be exercised:

            (a)   The corporate charter of Hoechst NewCo II will not be amended
                  in any material respect without the earlier written consent of
                  Hoechst which consent shall not be unreasonably withheld or
                  delayed except for (i) the amendments set forth in EXHIBIT
                  4.12(a) and (ii) any increases of Hoechst NewCo II's stated
                  capital in connection with a contribution in kind of all
                  shares in MGG held by MIG against shares and potentially a
                  loan receivable to compensate the Hoechst NewCo II
                  Contribution Receivable (not permitted are capital increases
                  through creation of GENEHMIGTES KAPITAL or BEDINGTES KAPITAL);
                  PROVIDED that as a re-

<Page>


                                      -35-

                  sult of such contribution by MIG the Hoechst NewCo II Option
                  Shares shall represent at least 66.2/3% of the registered
                  capital of Hoechst NewCo II. It is agreed that Investor NewCo
                  may procure a change of the fiscal year of Hoechst NewCo II;
                  PROVIDED that as of December 31, 2001, the fiscal year of
                  Hoechst NewCo II shall again be equal to the calendar year.

            (b)   Hoechst NewCo II shall not dispose of or encumber in any way
                  the Hoechst-MGG Share; PROVIDED that Hoechst NewCo II may
                  pledge or otherwise encumber the Hoechst-MGG Share with all
                  enforcement rights resulting therefrom as collateral for any
                  borrowings of Hoechst NewCo II, MGG or their Subsidiaries, and
                  further PROVIDED that Hoechst NewCo II may transfer its shares
                  in MGG to one or more of Hoechst NewCo II's directly or
                  indirectly wholly owned Subsidiaries which must remain wholly
                  owned until the expiration of the Counter-Put Option Period.

            (c)   Investor NewCo shall not dispose of or encumber in any way the
                  Hoechst NewCo II Option Shares.

            (d)   Hoechst NewCo II shall not enter into any agreement as
                  provided for in the German Transformation Act (UmwG) or be
                  liquidated, without the prior consent of Hoechst.

            (e)   If and to the extent Hoechst NewCo II makes dividend
                  distributions to Investor NewCo or to any other shareholder in
                  excess of profits generated during ordinary course of business
                  by itself, MGG or any of its Affiliates (e.g. profits
                  resulting from the disposal of interests directly or
                  indirectly held by Hoechst NewCo II), Investor NewCo shall
                  procure that any such excess dividends shall be recontributed
                  to the capital reserves of Hoechst NewCo II in accordance with
                  Section 272(2) No. 4 Commercial Code (HGB) on or prior to the
                  beginning of the Call Option Period; PROVIDED that such
                  recontribution can in any event be made in form of a note
                  (payment obligation) which will not be due for payment before
                  the expiration of the Counter-Put Option Period and, if the
                  Counter-Call Option or Counter-Put Option is exercised, not
                  before transfer of the Hoechst NewCo III shares to In-

<Page>


                                      -36-

                  vestor NewCo, and which note cannot be assigned, pledged or
                  otherwise encumbered (the "RECONTRIBUTION NOTE"), or by
                  contributing the Call Option Price or parts thereof.

            (f)   Investor NewCo shall not make any dividend distributions and
                  shall not grant any loans except to its Subsidiaries or to the
                  members of the management of such Subsidiaries or as
                  contemplated by the Singapore Separation Agreement; PROVIDED
                  that in connection with a contribution of MIG's newly created
                  interest in Hoechst NewCo II to Investor NewCo, Investor NewCo
                  may pay certain consideration to MIG as reflected in EXHIBIT
                  4.12(h).

            (g)   MGG shall not be converted into a different legal form.

            (h)   Investor NewCo shall as of the Closing Date and after giving
                  effect to the contributions of MIG to Investor NewCo be funded
                  as outlined in EXHIBIT 4.12(h).

            (i)   The debt of the Messer Group shall not be increased by more
                  than EUR 25,000,000 as a result of any dividend distributions
                  or loans granted to Investor NewCo and/or other direct or
                  indirect shareholders of Hoechst NewCo II, unless Investor
                  NewCo recontributes any such dividend distributions or repays
                  any such loans promptly in a situation where due to the
                  financial situation of the Messer Group a prudent businessman
                  would act accordingly.

            (j)   Investor NewCo and Hoechst NewCo II shall not enter into any
                  activities which would materially change the current core
                  business of the Messer Group (divestitures are permitted).

            (k)   Neither Investor NewCo, nor Hoechst NewCo II, nor MGG, nor any
                  of its Subsidiaries shall take any measure which would
                  endanger any existing or necessary future antitrust or other
                  regulatory approvals relating to the consummation of the
                  Hoechst NewCo II Share Purchase Agreement II.

            Any further restructuring and other measures of the MGG Group,
            including as set forth in EXHIBIT 4.12 or the disposals of
            Subsidiaries di-

<Page>


                                      -37-

            rectly or indirectly owned by MGG, is permitted and does not require
            any consent of Hoechst. Nevertheless, in the event of any material
            transactions directly and solely affecting Hoechst NewCo II or
            reorganizations of Hoechst NewCo II and not set forth in EXHIBIT
            4.12, Investor NewCo shall consult (without any consent requirement)
            with Hoechst in advance of any such transaction with a view to avoid
            potential damages of Hoechst or Hoechst NewCo III. Subject to
            Investor NewCo's obligation to comply with the covenants set forth
            in Sections 2.8 and 4.12, Investor NewCo does not assume any
            responsibility that the conditions on which the tax ruling for
            Hoechst are based are or will be met.

      4.13  COVENANTS UNDER CONTRIBUTION AGREEMENTS. Investor NewCo hereby
            agrees to procure that Hoechst NewCo II fulfills all of its
            obligations under the Hoechst NewCo II Contribution Agreement (e.g.,
            continuation of book value of the contributed Hoechst-MGG Share).

      4.14  NON-FULFILLMENT OF COUNTER-CALL.

            (a)   If Investor NewCo exercises the Counter-Call Option and, for
                  whatever reason and irrespective of any fault by Hoechst (OHNE
                  RUCKSICHT AUF VERTRETENMUSSEN) a 2002 Break-Up Situation
                  occurs, Hoechst shall have a right to cure the 2002 Break-Up
                  Situation, including all Losses resulting therefrom, within a
                  period of forty-five (45) Business Days after receipt of a
                  notification by Investor NewCo that contains a statement to
                  the effect that a 2002 Break-Up Situation has occurred and the
                  alleged Losses resulting therefrom. If Hoechst fails to cure
                  the 2002 Break-Up Situation including all Losses reasonably
                  evidenced in said notification, it shall pay a penalty
                  (VERTRAGSSTRAFE) to Investor NewCo in the amount of 200 % of
                  the Total Compensation (the "MINIMUM DAMAGE AMOUNT"), unless
                  and without affecting the validity or enforceability of any
                  Combination Document (i.e., solely for the purpose of the
                  Minimum Damage Amount) Hoechst provides for or the parties
                  agree in good faith on other transactional structures
                  providing, directly or indirectly, full legal and beneficial
                  ownership, as set forth in the Hoechst NewCo III

<Page>

                                      -38-


                  Share Purchase Agreement, of the Hoechst MGG Share and the
                  Hoechst NewCo II Option Shares to Investor NewCo or Affiliates
                  thereof in a manner not causing any damage or other detriment
                  to Investor NewCo; PROVIDED that the penalty represents the
                  agreed minimum damage (VEREINBARTER MINDESTSCHADEN) to which
                  Investor NewCo shall be entitled in such case and that the
                  payment of such minimum damage shall not preclude Investor
                  NewCo from requesting compensation for any higher damage
                  actually suffered; PROVIDED FURTHER that Investor NewCo shall
                  (upon receipt of the Minimum Damage Amount) re-transfer to
                  Hoechst the Hoechst NewCo III Shares together with the Hoechst
                  NewCo II Option Shares and the Hoechst MGG Share if and to the
                  extent previously received.

            (b)   "2002 BREAK-UP SITUATION" means a situation where Investor
                  NewCo after having exercised the Counter-Call Option, for
                  reasons other than the lack of antitrust or other regulatory
                  approvals fails to receive (and retain) full unencumbered
                  legal and beneficial ownership of the (A) Hoechst NewCo III
                  Shares, (B) with Hoechst NewCo III having such ownership of
                  the Hoechst NewCo II Option Shares and (C) Hoechst NewCo II
                  having such ownership of the Hoechst MGG Share, all as and
                  when contemplated by the relevant Combination Documents (B and
                  C except for any encumbrance or other limitation to ownership
                  which was in place when the Hoechst NewCo II Option Shares
                  were transferred to Hoechst NewCo III under the Call-Option
                  Agreement); PROVIDED that a 2002 Break-Up Situation shall not
                  be deemed to have occurred if and to the extent A, B or C have
                  not been satisfied as a result of a breach or non-fulfillment
                  (VERSCHULDENSUNABHANGIG) of any Combination Document by
                  Investor NewCo.

      4.15  NO CHANGE OF FORWARD SALE AGREEMENTS. Hoechst and Hoechst NewCo III
            shall procure that neither the Forward Sale Agreement I nor the
            Forward Sale Agreement II are amended in a material way or
            terminated without the prior written consent of Investor NewCo not
            to be unreasonably withheld or delayed.

<Page>


                                      -39-

      4.16  SPECIAL INDEMNIFICATION.

            (a)   In case of an exercise of the Call Option by Hoechst NewCo
                  III, Hoechst shall indemnify Investor NewCo for any positive
                  activities negatively affecting Hoechst NewCo II, MGG or any
                  of its Affiliates and performed by Hoechst NewCo III, Hoechst
                  or the Bank Sub as a shareholder of Hoechst NewCo II during
                  the period starting with the exercise of the Call Option and
                  ending at the earlier of (i) any transfer of the Hoechst NewCo
                  III Shares upon exercise of the Counter-Call Option or the
                  Counter-Put Option to Investor NewCo, as applicable, or (ii)
                  the expiration of the Counter-Put Option Period if neither the
                  Counter-Call nor the Counter-Put Option are exercised (the
                  "INDEMNIFICATION PERIOD").

            (b)   Hoechst shall indemnify Investor NewCo or Hoechst NewCo III
                  from any Income Tax liability arising from the granting of the
                  Call Option by Investor NewCo to Hoechst NewCo III.

            (c)   If MGG does not receive until December 31, 2001, either
                  directly or through its 49% interest in Messer Holding
                  Limited, ownership of an equity interest (including the rights
                  to exercise its shareholders' rights therein) of 50% minus one
                  in Bombay Oxygen Corporation Limited as contemplated pursuant
                  to a settlement with Goyal or an equivalent economical
                  interest under any new settlement, Hoechst shall indemnify
                  Investor NewCo for any Losses incurred by MGG up to an amount
                  of EUR 5,000,000; PROVIDED that Investor NewCo shall procure
                  that MGG or any of its Subsidiaries assigns to Hoechst upon
                  payment of any such indemnification for the lack of any direct
                  or indirect ownership in Bombay Oxygen Corporation Limited any
                  rights it may have against any Person in connection with or
                  relating to MGG's alleged shareholding in Bombay Oxygen
                  Corporation Limited. MGG shall strive to reasonably settle any
                  outstanding disputes with any Persons relating to the
                  foregoing and shall pursue its rights in court at the cost of
                  Hoechst upon reasonable request of Hoechst. Hoechst shall have
                  the right to fully control any such proceedings including to
                  select counsel at its choice.

<Page>

                                      -40-


            (d)   Hoechst shall indemnify Investor NewCo, subject to the Pro
                  Rata Principle, for any penalties imposed by any competent
                  anti-trust authorities upon the Messer Group Companies
                  specifically relating to (i) alleged illegal agreements for
                  the supply of industrial and medical gases against Messer
                  Nederland B.V. in the Netherlands, and (ii) the shareholding
                  of Messer Austria GmbH in Klara Kohlensaure Ges.m.b.H. & Co.
                  KG; PROVIDED that (i) Investor NewCo shall procure that
                  Hoechst shall be in a position to fully control at its own
                  cost such proceedings initiated by the anti-trust authorities
                  and subsequent court proceedings including the filing of any
                  objections (e.g., Hoechst shall be entitled to propose any
                  outside counsel representing the interests of MGG), and (ii)
                  Investor NewCo shall procure that MGG shall take any
                  reasonable steps to mitigate the amount of any penalties
                  possibly imposed by the anti-trust authorities and shall
                  cooperate to provide all information necessary or helpful to
                  conduct such proceeding.

            (e)   Hoechst shall indemnify Investor NewCo, subject to the Pro
                  Rata Principle, for any Losses arising out of or relating to
                  the investigation referred to in EXHIBIT D and any related
                  proceedings, other than reasonable attorney's fees at
                  aggregate levels comparable to those incurred by MGG in
                  connection with such investigation during the four-month
                  period prior to the date of the execution of this Agreement.

            (f)   Hoechst shall indemnify MGG against any Losses MGG has
                  incurred or will incur after the Financial Date in respect of
                  any liability towards Mr. Ruia or any other current or
                  previous shareholder of Bombay Oxygen Corporation Limited or
                  their legal successors, stemming from any misrepresentation
                  with respect to the shareholding structure of Messer Holding
                  Limited, British Virgin Islands, if any such misrepresentation
                  has occurred. Any indemnification payable by Hoechst under
                  this Section 4.16(f) shall be subject to the provisions set
                  forth in Section 6.

            (g)   Hoechst shall indemnify MGG against 1/3 of any Losses MGG or
                  MG Industries may incur in respect of certain alleged claims
                  of MG Systems & Welding Inc. against MG Industries relating to

<Page>


                                      -41-

                  intra-group receivables and a possible overpayment of a
                  purchase price for MG Systems & Welding Inc., which claims
                  amount to an aggregate amount of US$ 2,500,000 (i.e., the
                  maximum liability of Hoechst hereunder amounts to
                  US$ 833,000), if and to the extent such claims of MG
                  Systems & Welding Inc. prove to be valid and enforceable
                  (if reasonably requested by Hoechst, to be determined by a
                  non-appealable court-decision.)

      4.17  DISPROPORTIONATE GUARANTEE INDEMNIFICATION.

            (a)   COVERED ENTITIES. EXHIBIT 4.17 shows, as of August 31, 2000,
                  the Financial Guarantees granted by MGG or any direct or
                  indirect Subsidiaries of MGG with respect to debt of Non-Fully
                  Owned Messer Group Companies and certain entities which are no
                  longer owned by MGG (collectively the "COVERED ENTITIES")
                  where the portion of the debt guaranteed by an MGG Guarantor
                  exceeds MGG's direct or indirect percentage equity interest,
                  if any, in the Covered Entity (the "DISPROPORTIONATE GUARANTEE
                  SCHEDULE"). Each such guarantee is referred to as a "COVERED
                  GUARANTEE".

            (b)   GENERAL RULE. With respect to the Covered Guarantees and the
                  apportionment of any claims of any holder of such Covered
                  Guarantees (the "GUARANTEE HOLDER"), solely the rules of this
                  Section 4.17 shall apply, excluding all other provisions of
                  this Agreement with respect to indemnifications,
                  representations and warranties in respect of any such
                  guarantees. The apportionment of claims under Covered
                  Guarantees shall distinguish between four different classes of
                  Covered Entities and shall take into regard the percentage of
                  MGG's direct or indirect equity interest in such entities as
                  well as the portion of Financial Debt secured by any Covered
                  Guarantee as follows:

            (c)   DEFINITIONS.

                  (i)   MGG or the Subsidiary of MGG which is the guarantor
                        under a Covered Guarantee is referred to as a "MGG
                        GUARANTOR".

<Page>


                                      -42-

                  (ii)  As to each Covered Entity, the Disproportionate
                        Guarantee Schedule indicates whether such Covered Entity
                        is deemed to be a Controlled Covered Entity, a Special
                        Non-Controlled Covered Entity, a Sold Covered Entity or
                        an Other Non-Controlled Covered Entity.

                  (iii) As to each Covered Guarantee, the Disproportionate
                        Guarantee Schedule sets forth the maximum exposure of
                        the MGG Guarantor under such Covered Guarantee (the
                        "MAXIMUM GUARANTEED AMOUNT"), which in the case of a
                        Covered Guarantee relating to a Controlled Covered
                        Entity or an Other Non-Controlled Covered Entity is
                        deemed for purposes hereof to be equal to the amount of
                        debt outstanding as of August 31, 2000 and then
                        guaranteed by such Covered Guarantee (the "FINANCIAL
                        DATE EXPOSURE"), and which in the case of a Covered
                        Guarantee relating to a Special Non-Controlled Covered
                        Entity or a Sold Covered Entity will equal the amount of
                        debt that would be outstanding if the debt then
                        guaranteed by such Covered Guarantee had been drawn
                        down in full (the "TOTAL EXPOSURE").

                  (iv)  MGG's direct or indirect percentage equity interest, if
                        any, in each Covered Entity as of August 31, 2000 shall
                        be the "MGG PERCENTAGE"; as to any Financial Debt that
                        is subject to a Covered Guarantee the portion (expressed
                        as a percentage) of the total amount of such Financial
                        Debt which is subject to the Covered Guarantee shall be
                        the "GUARANTEE PERCENTAGE"; the amount (expressed as a
                        percentage) by which the Guarantee Percentage exceeds
                        the MGG Percentage shall be the "EXCESS PERCENTAGE"; and
                        the "ADJUSTED EXCESS PERCENTAGE" shall be the Excess
                        Percentage divided by the Guarantee Percentage For
                        example: If a Covered Guarantee is with respect to 90%
                        of a loan of a 60% Subsidiary, the Excess Percentage
                        would be 30% (90%-60%), and the Adjusted Excess
                        Percentage would be 33 1/3% (30% /. 90).

<Page>

                                      -43-


                  (v)   The "COVERED PERIOD" means, (A) with respect to a
                        Covered Guarantee relating to a Controlled Covered
                        Entity, the period between August 31, 2000 and the date
                        which is 18 months following the Closing Date, (B) with
                        respect to a Covered Guarantee relating to a
                        Non-Controlled Entity, the period between August 31,
                        2000 and the date which is 24 months following the
                        Closing Date, and (C) with respect to the Covered
                        Guarantees of the Special Non-Controlled Covered
                        Entities, the period between August 31, 2000 and the
                        date which is 24 months following the Closing Date, (D)
                        with respect to a Sold Covered Entity the period between
                        August 31, 2000 and the date which is 18 months
                        following the Closing Date; PROVIDED that in any event
                        for each Covered Entity (except for the Sold Covered
                        Entities) the Covered Period shall end on the day on
                        which more than 50 % of MGG's direct or indirect
                        interest as of the Closing Date in such Covered Entity
                        is transferred to a third party which is not a
                        Subsidiary of MGG or Investor NewCo, unless such
                        transfer is the result of a drag-along right or a
                        similar right exercised by a Person holding an equity
                        interest in the Covered Entity.

            (d)   NOTICE OBLIGATION. In the event that, at any time during the
                  Covered Period, an MGG Guarantor receives any notice or demand
                  from the holder of the Covered Guarantee indicating that it
                  is obligated to make any payment of any nature pursuant to or
                  in respect of a Covered Guarantee (a "GUARANTEE DEMAND
                  NOTICE"), Investor NewCo shall procure and MGG shall promptly
                  notify Hoechst of the receipt of such Guarantee Demand Notice
                  and provide Hoechst with a copy thereof. As promptly as is
                  practicable thereafter, Investor NewCo shall procure and MGG
                  shall provide Hoechst with notice (a "PAYMENT NOTICE") of the
                  total amount due and to be paid by the MGG Guarantor pursuant
                  to such Guarantee Demand Notice (the "TOTAL MGG PAYMENT") and
                  the date upon which such amount is to be paid by the MGG
                  Guarantor (the "MGG PAYMENT DATE").

<Page>


                                      -44-

            (e)   PAYMENTS UNDER COVERED GUARANTEES. On the MGG Payment Date,
                  Hoechst shall pay to the Guarantee Holder, on behalf of MGG
                  and in partial satisfaction of MGG's obligation under the
                  related Covered Guarantee, by wire transfer of immediately
                  available funds to an account of the Guarantee Holder
                  designated in the Payment Notice an amount (the "HOECHST
                  PAYMENT AMOUNT") equal to 66 2/3 % of the Adjusted Excess
                  Percentage of the lesser of (i) the Total MGG Payment or (ii)
                  the Maximum Guaranteed Amount; PROVIDED that if the Covered
                  Guarantee relates to a Special Non-Controlled Entity the
                  Hoechst Payment shall not exceed the sum of 66 2/3% of the
                  Adjusted Excess Percentage of the Financial Date Exposure plus
                  50% of the Adjusted Excess Percentage of the difference
                  between the Total Exposure and the Financial Date Exposure.
                  The foregoing is summarized for clarification purposes in the
                  chart attached as a supplement to EXHIBIT 4.17; PROVIDED that
                  in case of any conflict, the provisions of this Section 4.17
                  shall prevail.

                  Hoechst shall only be obliged to pay the Hoechst Payment
                  Amount if Investor NewCo and MGG have provided Hoechst with a
                  written confirmation of MGG

                  (i)   that it believes in good faith after due inquiry that
                        the Covered Entity (in case of a Controlled Covered
                        Entity) does not have sufficient cash or cash
                        equivalents at hand or cash that can be drawn from
                        existing credit lines to fulfill its payment obligations
                        against the Guarantee Holder, and

                  (ii)  of MGG that the MGG Guarantor is paying on the MGG
                        Payment Date the difference between the Total MGG
                        Payment and the Hoechst Payment Amount to the Guarantee
                        Holder.

                  Hoechst may delay such payment to the Guarantee Holder to the
                  extent that there is less than ten (10) Business Days between
                  de-

<Page>

                                      -45-


                  livery of the Payment Notice and the Payment Date (the "10
                  BUSINESS DAYS DELAY RULE").

            (f)   SET-OFF, COUNTERCLAIMS AND INTEREST. The payments required to
                  be made by Hoechst to any Guarantee Holder pursuant to the
                  provisions of this Section 4.17 shall not be subject to any
                  rights of set off or counterclaim between Hoechst, MGG and
                  Investor NewCo, and if not made when due shall bear interest
                  at a rate equal to the rate (including any penalty) payable by
                  MGG to the Guarantee Holder due to the non-payment of Hoechst
                  (the "APPLICABLE RATE") and such interest shall also be paid
                  directly from Hoechst to the Guarantee Holder; PROVIDED that
                  the foregoing shall not be deemed to limit any right of
                  Investor NewCo to claim damages for any breach by Hoechst of
                  its payment obligations under this Section 4.17 and provided
                  further that MGG shall indemnify Hoechst for any interest to
                  be payable to the Guarantee Holder and being paid by Hoechst
                  to the Guarantee Holder as a consequence of the 10 Business
                  Days Delay Rule. It is understood that the MGG Guarantor will
                  raise any material objections or defenses it determines in
                  good faith to have and reasonably likely to prevail in court
                  or otherwise with respect to its obligations under the Covered
                  Guarantee.

            (g)   DISPUTES REGARDING PAYMENT OBLIGATION. In the event of any
                  dispute between MGG and Hoechst relating to the obligation of
                  Hoechst to pay any Hoechst Payment Amount, Hoechst shall
                  notwithstanding such dispute make payment to the relevant
                  Guarantee Holder of such Hoechst Payment Amount in full
                  subject to the provisions above, and subject to the right of
                  Hoechst to obtain from such MGG Guarantor repayment thereof
                  plus interest thereon at the Applicable Rate in the event that
                  it is determined that such payment was not required to be made
                  by Hoechst.

            (h)   CONSEQUENCES OF PAYMENT WITH RESPECT TO SECURED DEBT. Upon
                  payment by Hoechst, (i) Investor NewCo and MGG shall procure,
                  if legally possible, that either the Guarantee Holder is
                  subrogated by Hoechst or the Guarantee Holder assigns directly
                  to Hoechst that part of its payment claim against the Covered
                  Entity

<Page>


                                     -46-

                  which equals the Hoechst Payment Amount, and (ii) Investor
                  NewCo shall procure and MGG shall assign, both if legally
                  possible, that part of its payment claims against the Covered
                  Entity which results from the payment of the Hoechst Payment
                  Amount, if any.

            (i)   STANDSTILL. Hoechst agrees for a period of twenty-four (24)
                  months after the MGG Payment Date but not for longer than
                  thirty-six (36) months after the Closing Date (the "STANDSTILL
                  PERIOD") not to pursue or enforce its payment claims against
                  the Covered Entity other than as a part of and in conjunction
                  with any such pursuit or enforcement by MGG. It is further
                  agreed that to the extent MGG or any Affiliate of MGG injects
                  new capital (either as equity or shareholder loans) to the
                  Covered Entity after the MGG Payment Date, such new capital
                  shall be as between Hoechst and MGG or the applicable MGG
                  Affiliate senior to the claims of Hoechst against the Covered
                  Entity in the event (and only then) that as a result of or in
                  connection with any liquidation, insolvency, bankruptcy, sale
                  of all or substantially all assets or similar extraordinary
                  transactions or events Hoechst receives any amount in respect
                  of such claims in advance at a time when MGG or its applicable
                  Affiliate is not receiving any repayment of such new capital,
                  then Hoechst shall pay over to MGG or its applicable Affiliate
                  all or the appropriate portion of such amount received by
                  Hoechst. With respect to any Covered Entity, the Standstill
                  Period shall end on the day on which more than 50 % of MGG's
                  direct or indirect equity interest as of the Closing Date in
                  such Covered Entity is transferred to a third party which is
                  not an Affiliate of MGG or Investor NewCo, unless such
                  transfer is the result of a drag-along right or a similar
                  right exercised by a Person holding an equity interest in the
                  Covered Entity.

            (j)   INFORMATION RIGHTS. During the Covered Period, MGG will use
                  its reasonable best efforts to keep Hoechst informed on a
                  current basis with respect to and to consult with Hoechst
                  regarding any material developments relating to a Covered
                  Entity that MGG

<Page>

                                     -47-


                  expects are likely to result in its receipt of a Guarantee
                  Demand Notice; in addition, MGG will provide Hoechst with
                  information on the equity and available funds of the Covered
                  Entities to the extent reasonably requested by Hoechst from
                  time to time.

            (k)   LEGAL ACTIONS. In the event that MGG or another MGG Guarantor
                  wishes to pursue any legal action contesting any Guarantee
                  Demand Notice or the right of any creditor of a Covered
                  Entity to deliver a Guarantee Demand Notice, Hoechst shall
                  bear the costs in the proportion of the Hoechst Payment Amount
                  to the Total MGG Payment; PROVIDED that in such case Hoechst
                  shall have the right to participate in such proceedings to the
                  extent legally possible.

            (l)   AMENDMENTS OF DISPROPORTIONATE GUARANTEE SCHEDULE. In the
                  event that, at any time following the execution of this
                  Agreement,

                  (i)   Investor NewCo becomes aware of any obligation, in
                        existence as of August 31, 2000, of MGG or any direct
                        or indirect Subsidiary of MGG to guarantee for Financial
                        Debt of any Non-Fully Owned Messer Group Company (or any
                        entity no longer directly or indirectly owned by MGG)
                        that is either not set forth in the Disproportionate
                        Guarantee Schedule or incorrectly reflected on the
                        Disproportionate Guarantee Schedule and

                  (ii)  Where, in the case of any Non-Fully Owned Messer Group
                        Company, as of August 31, 2000, the portion of the debt
                        so guaranteed (or supported) exceeded the direct or
                        indirect percentage equity interest of MGG in such
                        Non-Fully Owned Messer Group Company,

                  Investor NewCo shall be entitled, by delivery of written
                  notice to Hoechst, to amend the Disproportionate Guarantee
                  Schedule to properly include such obligation. Such amendment
                  shall be deemed to have been effective as of the date of the
                  execution of this Agreement.

<Page>

                                      -48-


                  In the event that any letter of comfort identified in EXHIBIT
                  4.17 is determined to constitute a legally binding obligation
                  of MGG or any direct or indirect subsidiary of MGG, such
                  obligation may be treated by Investor NewCo in the same manner
                  as an obligation referred to above in this Section (l);
                  PROVIDED that, to the extent legally possible, Hoechst shall
                  have the right to participate in any legal proceedings
                  regarding the determination of the question of whether any
                  such letter of comfort constitutes a legally binding
                  obligation.

            (m)   At the Closing Date, Hoechst shall pay to MGG or the MGG
                  Guarantor in cash an amount equal to the aggregate amount of
                  all payments that would have been payable by Hoechst in
                  respect of Covered Guarantees pursuant to this Section 4.17 at
                  any time subsequent to August 31, 2000 if the Closing Date had
                  been August 31, 2000, and MGG or the MGG Guarantor to be
                  procured by MGG shall assign to Hoechst the appropriate
                  portion of its related payment claim against the applicable
                  Covered Entities upon receipt of the respective payment of
                  Hoechst by MGG.

            (n)   It is agreed that between Hoechst, MGG and MIG any payments
                  made by Hoechst to the Guarantee Holder or to the MGG
                  Guarantor under this Section 4.17 shall be made for the
                  benefit of Investor NewCo as for the internal relationship
                  between MIG and the other shareholders of Investor NewCo.

      4.18  MONETARIZATION OF LOAN NOTES. Investor NewCo shall use reasonable
            endeavors to obtain, prior to the Closing Date, one or more
            commitments by one or more internationally recognized financial
            institutions, which may include a German Allianz insurance company,
            to acquire the Loan Notes for face value without recourse at or
            immediately after the Closing Date.

      4.19  SINGAPORE.

            (a)   If and to the extent Texaco Netherland B.V. and/or Dresdner
                  Bank as facility agent for all relevant banks financing
                  Singapore

<Page>


                                      -49-

                  Pte. Ltd. have not consented to the consummation of the
                  Singapore Separation Agreement prior to the Closing Date, the
                  parties shall place each other for the time between the
                  Closing Date and the date when such consents are finally
                  granted, economically in the same position as if the Singapore
                  Separation Agreement had been consummated; PROVIDED that the
                  parties shall continue to use reasonable best efforts to
                  obtain any required consents to consummate the Singapore
                  Separation Agreement.

            (b)   If and to the extent the consents referred to in Section
                  4.19(a) can only be obtained upon making adjustments to the
                  Singapore Separation Agreement, the parties shall in good
                  faith consider such adjustments. In any event, Hoechst, MGG
                  and Investor NewCo are prepared to give separate guarantees to
                  Texaco that they or their appropriate Affiliates will comply
                  with their funding obligations under the Singapore Separation
                  Agreement.

      4.20  CONTRIBUTIONS BY INVESTOR NEWCO. To the extent Investor NewCo
            receives indemnifications from Hoechst NewCo III or Hoechst under
            this Agreement (e.g. pursuant to Sections 4, 6, 8 or 9), it shall
            forward any such indemnifications into the capital or capital
            reserves (with or without consideration) to MGG or grant such
            indemnity to MGG as a subordinated shareholder loan (RANGRUCKTRITT
            GEGENUBER ALLEN ANDEREN GLAUBIGERN). Such obligation shall expire on
            February 28, 2002.

      4.21  ADDITIONAL HOECHST SHAREHOLDING IN MESSER EGYPT S.A.E.. If and to
            the extent Hoechst owns any shares in Messer Egypt S.A.E., it shall
            contribute those shares, upon request of Investor NewCo, into the
            capital reserves of MGG or transfer it to any designee of MGG
            without any consideration. The consummation of any such contribution
            shall not be deemed to be a closing condition under Section 3.1.

5.    REPRESENTATIONS AND WARRANTIES OF HOECHST

      Hoechst hereby represents and warrants in the meaning of an independent
      guarantee to Investor NewCo as of the Closing Date or such other date
      specified hereinafter, excluding any other statutory warranty claim, that
      the following is

<Page>


                                      -50-

      true and correct, whereby it is understood that any disclosures of events
      or documents which (i) are below any materiality threshold set forth in
      the respective representations, or (ii) contain additional information not
      relevant for the respective representations, all with respect to a
      specific disclosure to which such information relate (unless the
      disclosure obviously and from its face also affects another
      representation), shall not be deemed to alter or in any way affect the
      contents of such guarantee, in particular in respect of the materiality
      thresholds:

      5.1   MESSER GROUP. Hoechst NewCo II is the owner of the Hoechst MGG Share
            which represents 66 2/3% of the registered capital of MGG and ACIC
            is the owner of all China Interests. EXHIBIT C contains as of the
            date of the notarization of this Agreement a correct list of all
            Persons except for the Participations in which MGG holds a direct or
            indirect shareholding, voting securities or other ownership
            interests (including without limitation silent participations)
            (collectively "INTERESTS") and the direct or indirect percentage of
            such Interest; PROVIDED that such list will be correctly updated as
            of the Closing Date to reflect changes after the notarization of
            this Agreement.

            "PARTICIPATIONS" means unless it is a Material Messer Company (i)
            Interests held by MGG as current assets (UMLAUFVERMOGEN), (ii)
            Interests held by a Person in which MGG directly or indirectly holds
            an Interest of less than 50%, (iii) Interests of less than 25% held
            by a Person in which MGG directly or indirectly holds an Interest
            exceeding 50%.

            Each Messer Group Company (except for the Participations) and each
            Material Messer Company is duly incorporated and validly existing
            under the laws of its respective jurisdiction of incorporation and
            has all requisite corporate power and authority to own or lease its
            properties and assets and to carry out its business. As of the date
            hereof, no bankruptcy or composition proceedings against any of the
            Messer Group Companies (except for the Participations) or any of the
            Material Messer Companies has been applied for nor has such
            application been rejected due to lack of assets nor are there any
            circumstances which would compel the initiation of such proceedings.
            The articles, by-laws and commercial register registrations of the
            Material Messer Companies in force as of August 31,

<Page>

                                     -51-


            2000 (including amendments which have not been registered) (the
            "CORPORATE DOCUMENTS") have been delivered to Investor NewCo before
            the notarization of this Agreement, and a correct list of which is
            set forth in EXHIBIT 5.1 and such Corporate Documents will be kept
            in custody by the officiating notary until June 30, 2002. The
            officiating notary is hereby instructed to grant access to such
            Corporate Documents to Hoechst and Investor NewCo and to provide
            copies to each of them at their respective cost, all upon their
            respective request. After June 30, 2002, the officiating notary
            shall deliver such Corporate Documents to a Person as jointly
            instructed by Hoechst and Investor NewCo.

      5.2   CAPITAL STOCK. The authorized and outstanding capital stock of the
            Messer Group Companies and all Material Messer Companies (the
            "MESSER CAPITAL STOCK") is duly authorized, validly issued and, to
            the extent legally required, and subject to Section 5.14 paid in.
            The registered capital of MGG is fully paid in. Except as agreed in
            this Agreement or otherwise agreed between the parties hereto, there
            are no issued or outstanding warrants, options, "phantom" stock
            rights, agreements, convertible or exchangeable securities or other
            commitments (except for the plans described in Section 5.10(a)(i),
            (ii) or in Section 5.12) pursuant to which any of the Material
            Messer Companies is or may become obligated to issue, sell,
            purchase, return or redeem any of the Messer Capital Stock. The
            statements made in this Section 5.2 do not apply to the
            Participations.

      5.3   NO BREACH. Except as set forth in EXHIBIT 5.3, the execution,
            delivery and performance by Hoechst, Hoechst NewCo III, ACIC (to the
            extent it is a seller under the China SPA) and MGG of this Agreement
            and the other Combination Documents does not and the consummation of
            the transactions contemplated herein and therein will not conflict
            with or result in a breach of the terms, conditions or provisions of
            or constitute a default under their certificate of incorporation or
            by-laws or other corporate instrument to which they are subject.

      5.4   NO ENCUMBRANCES ON STOCK. The Messer Capital Stock owned by any
            Material Messer Company is free of any pledges, attachments,
            usufruct,

<Page>

                                     -52-


            transfers for security purposes or similar security charges
            (collectively "ENCUMBRANCES") except for Encumbrances (i) which
            relate to debt reflected in the accounts of the respective Material
            Messer Companies on August 31, 2000 or incurred thereafter in the
            ordinary course of business, or (ii) which are imposed by Applicable
            Law. Subject to Section 12.1, the Hoechst MGG Share is free of any
            Encumbrances.

      5.5   FINANCIAL STATEMENTS. The Financial Statements 1999 have been
            prepared in accordance with Applicable Law and IAS as consistently
            applied and present fairly in all material respects the financial
            condition, results of operations and cash flows of the Gas Business
            in accordance with IAS for the period ending December 31, 1999,
            except for those measures listed in EXHIBIT 5.5. Furthermore, the
            audited individual financial statements of such Material Messer
            Companies which have been consolidated in the Financial Statements
            1999 have been prepared in accordance with Applicable Law and
            present fairly in all material respects the financial conditions,
            results of operations and cash flows of their business in accordance
            with such respective Applicable Law for the period ending on
            December 31, 1999. The books and records of the Material Messer
            Companies have been in all material respects fully, properly and
            accurately kept, do not contain any material inaccuracies or
            material discrepancies and are available at the offices of the
            respective Material Messer Companies.

      5.6   ORDINARY COURSE OPERATION. Except as set forth in EXHIBIT 5.6 or as
            contemplated by this Agreement, since January 1, 2000, the Material
            Messer Companies have been operated in the ordinary course of
            business and there has not been, all unless otherwise agreed by
            Investor NewCo,

            (a)   any waiver of any material right belonging to the Material
                  Messer Companies which is either indispensable for any
                  material portion of the Gas Business or which waiver was made
                  without proper compensation;

            (b)   to Hoechst's best knowledge, unless provided for in this
                  Agreement or disclosed in any Exhibit of this Section 5, as of
                  the date hereof, any extraordinary one-time event having a
                  negative effect

<Page>

                                     -53-


                  in the consolidated financial statements for the year 2000 or
                  otherwise materializing in the year 2000, of at least EUR
                  5,000,000 in each individual case, on the Messer Group as a
                  whole; or

            (c)   conclusion or termination of any non-arm's-length agreement
                  with Aventis and its subsidiaries (other than the Messer
                  Group).

      5.7   REAL PROPERTY.

            (a)   OWNED PROPERTIES. EXHIBIT 5.7(a)(1) attached hereto contains a
                  list of all real estate (including ERBBAURECHTE) owned by the
                  Material Messer Companies as of August 31, 2000 (the "MATERIAL
                  MESSER OWNED REAL PROPERTY"). Except as set forth in EXHIBIT
                  5.7(a)(2), with respect to each parcel of Material Messer
                  Owned Real Property, immediately after giving effect to the
                  Closing, the respective Material Messer Companies shall have
                  good and marketable title to all such parcels, free and clear
                  of all Liens, except Permitted Liens and except for
                  transactions made after August 31, 2000 in the ordinary course
                  of business.

            (b)   LEASED PROPERTIES. EXHIBIT 5.7(b) attached hereto contains all
                  real estate leased by the Material Messer Companies as of
                  August 31, 2000 involving annual payments in excess of EUR
                  300,000 in each individual case.

            (c)   CONDITION OF IMPROVEMENTS. Except as set forth in EXHIBIT
                  5.7(c), all buildings, structures, fixtures and other
                  improvements and all components thereof (the "IMPROVEMENTS")
                  included within the real property described in subsections (a)
                  and (b) above are, in all material respects, in good
                  condition and repair, ordinary wear and tear accepted.

            It is agreed and acknowledged that the Exhibits relating to this
            Section 5.7 do not contain appropriate information with respect to
            Material Messer Companies located in South Africa and that such
            inappropriate information does not constitute a breach of the
            guarantees contained in this Section 5.7; PROVIDED that Hoechst
            hereby guarantees that all real estate and improvements used by
            Material Messer Companies in South

<Page>

                                      -54-


            Africa are either owned by such companies or leased on the basis of
            lease agreements which have a remaining term of not less than two
            (2) years from the Closing Date and the terms of which are
            substantially consistent with current market terms.

      5.8   GAS BUSINESS ASSETS. The Material Messer Companies economically own
            (i) as of December 31, 1999 the fixed tangible assets as reflected
            in the Financial Statements 1999, and (ii) as of the date hereof the
            fixed tangible assets as reflected in the consolidated interim
            financial statements of the Messer Group as of June 30, 2000, which
            have been submitted to Investor NewCo prior to the date hereof,
            except for disposals by the Material Messer Companies in the
            ordinary course of business after June 30, 2000 and except for
            retentions of title and other customary rights of third parties,
            such fixed tangible assets are free and clear of any Liens, with the
            exception of Permitted Liens, all unless stated otherwise in EXHIBIT
            5.8. The Material Messer Companies either own or have a right to use
            all fixed tangible assets (ANLAGEVERMOGEN) reasonably required to
            conduct their business as presently conducted. Those assets are in
            proper working condition fit to be used for their respective
            purposes, normal wear and tear accepted. The guarantees contained in
            this Section 5.8 are made as of the date hereof.

      5.9   INTELLECTUAL PROPERTY RIGHTS. The Material Messer Companies are
            registered holders of all patents, trademarks and other registered
            intellectual property rights (GEWERBLICHE SCHUTZRECHTE) listed in
            EXHIBIT 5.9 subject to any licenses or other rights granted to third
            parties, and to Hoechst's best knowledge as of the date hereof the
            processes used and products manufactured, distributed and sold in
            the Gas Business do not use (i.e. infringe) in a manner material to
            the Gas Business any intellectual property rights and/or know-how in
            each case owned by any third parties other than on the basis of oral
            or written agreements or except as listed in EXHIBIT 5.9. Except as
            listed in EXHIBIT 5.9, third parties have not made any written
            allegations to a Material Messer Company as to any infringements by
            such Material Messer Company. All necessary fees have been paid,
            renewals made and other necessary steps taken for the maintenance of
            each of the intellectual property rights listed in EXHIBIT 5.9. To
            Hoechst's best knowledge except as disclosed in EXHIBIT 5.9 as of
            the

<Page>

                                      -55-


            date hereof no third party is in a material violation of any of the
            intellectual property rights owned by the Material Messer Companies.
            The guarantees contained in this Section 5.9 are made as of the date
            hereof.

      5.10  CONTRACTS AND COMMITMENTS.

            (a)   Except as set forth in EXHIBIT 5.10(a)(i) through EXHIBIT
                  5.10(a)(xiii) (which Exhibit indicates the pertinent parties
                  thereto and the dates and amendment dates, if any), with
                  respect to the Gas Business, no Material Messer Company is on
                  the date hereof a party to any oral or written:

                  (i)   contract providing for bonuses, options, stock
                        purchases, deferred compensation or profit sharing in
                        any such case reasonably to be expected in excess of
                        EUR 100,000 for a calendar year up to the year 2002;

                  (ii)  plans covering groups of employees providing for
                        bonuses, options, stock purchases, deferred compensation
                        or profit sharing;

                  (iii) contract with respect to the lending or borrowing of
                        funds (no finance leases) involving more than EUR
                        2,500,000 in the aggregate and guarantees or similar
                        enforceable contractual obligations securing such
                        borrowings (except for aval) by the Material Messer
                        Companies;

                  (iv)  guarantee or any similar enforceable contractual (except
                        for aval) obligation involving more than EUR 2,500,000
                        in the aggregate for any obligations of any Persons;

                  (v)   license or royalty agreement other than listed in
                        EXHIBIT 5.9 involving payments of more than EUR
                        1,250,000 per year;

                  (vi)  contract or group of related contracts with the same
                        party for the purchase or sale of commodities, supplies,
                        products or other personal property or for the
                        furnishing or re-

<Page>

                                     -56-


                        ceipt of services having a sales price in excess of
                        EUR 10,000,000 per year;

                  (vii) contract not otherwise disclosed in EXHIBIT 5.10 and
                        containing a payment obligation of a Material Messer
                        Company in an individual year in excess of EUR
                        10,000,000 until the year of its earliest termination by
                        the respective Material Messer Company;

                  (viii)contract that would prohibit any Material Messer
                        Company from freely engaging in the Gas Business
                        anywhere in the world except for prohibitions resulting
                        from the agreements or documents listed in EXHIBIT 5.1,
                        EXHIBIT 5.10(a)(v), (vi) OR (x) or resulting from
                        customary distributorship or agency agreements which
                        have been entered into in the ordinary course of
                        business;

                  (ix)  contract with Aventis and its subsidiaries (other than
                        the Messer Group) other than the delivery of services
                        and goods at arm's length conditions;

                  (x)   joint venture agreement, partnership agreement or
                        shareholder agreement, all to the extent relating to the
                        joint shareholding in a Messer Group Company and not
                        already listed in EXHIBIT 5.1, such listed agreements
                        also to be regarded as Corporate Documents, except for
                        those entered into by this Agreement or waived pursuant
                        to Section 12.1 of this Agreement. To the extent joint
                        venture agreements are not disclosed in EXHIBIT
                        5.10(a)(x) or other Corporate Documents have not been
                        delivered prior to the execution of this Agreement,
                        Investor NewCo accepts that the relevant joint venture
                        partner of a Material Messer Company has a customary
                        right of first offer.

                  (xi)  company agreement in the meaning of Section 291 et seq.
                        of the German Stock Corporation Act;

                  (xii) contract involving financial derivatives (i.e. financial
                        options, futures and forwards, interest swaps and
                        currency

<Page>

                                      -57-


                        swaps, which create rights and obligations that have the
                        effect of transferring between the parties to such
                        instrument one or more of the financial risks inherent
                        in an underlying primary financial instrument and which
                        do not result in a transfer of the underlying primary
                        financial instrument on such inception of the contract);

                  (xiii) agreement with third parties providing for preemptive
                        rights, rights of first refusal, option rights, rights
                        of first offer or other rights to acquire any shares in
                        any Material Messer Company, except if the respective
                        agreement or document is already disclosed in EXHIBIT
                        5.1 OR EXHIBIT 5.10(a)(x).

            (The documents referred to in (i) through (xiii) collectively the
            "MATERIAL MESSER CONTRACTS").

            It is agreed that with respect to any items guaranteed under Section
            5.1(a)(iii) but only to the extent relating to lending or
            borrowing of funds, the content of a report dated October 13, 2000
            and called "Review Procedures on Net Debt Position and Working
            Capital for the Messer Griesheim Group", and attached hereto as
            EXHIBIT 5.10(a)(a) for evidence purposes only, is deemed to be
            disclosed hereunder but only to the extent amounts of debt are
            disclosed in such report; PROVIDED that for any excess amounts the
            aforementioned guarantees remain unaffected.

            (b)   Except as set forth in EXHIBIT 5.10(b), as of the date hereof

                  (i)   none of the Material Messer Contracts has been
                        terminated by any contract party, nor has any such
                        contract party given written notice about its intention
                        to terminate a Material Messer Contract;

                  (ii)  no Material Messer Company is to the respective Material
                        Messer Company's top management's best knowledge in
                        default as to the fulfillment of any material obligation
                        arising from a Material Messer Contract;

<Page>

                                      -58-


                  (iii) no Person has stated in writing to a Material Messer
                        Company that a Material Messer Contract is partially or
                        entirely invalid.

                  Furthermore, except as set forth in EXHIBIT 5.10(b), to
                  Hoechst's best knowledge as of the date hereof the Material
                  Messer Contracts are in full force and effect.

      5.11  LITIGATION. Except as listed in EXHIBIT 5.11, there are no actions,
            suits, proceedings, orders or investigations pending or, to
            Hoechst's best knowledge as of the date hereof, threatened against
            or affecting any of the Material Messer Companies at law, or in
            equity, or before or by any Governmental Entity involving amounts in
            dispute in excess of EUR 1,000,000. The guarantees contained in this
            Section 5.11 are made as of the date hereof.

      5.12  EMPLOYEES. EXHIBIT 5.12(a) lists the corporate function and the
            dates of the employment agreement of individuals employed by certain
            Material Messer Companies having an annual base salary in excess of
            EUR 125,000 (the "KEY EMPLOYEES") and individuals engaged as
            consultants (the "CONSULTANTS") who receive a yearly consultancy fee
            in excess of EUR 125,000. None of the Key Employees has terminated
            his/her employment agreement. EXHIBIT 5.12(b) lists all internal
            collective employee agreements (i.e. agreements which are entered
            into between any Material Messer Company and a group of employees or
            a representative body of employees of the Material Messer Companies
            unless such agreements not only substantially repeat or reflect
            mandatory statutory law), which contain

            (a)   benefit or incentive plans relating to or triggered by a
                  change-of-control of the respective Material Messer Company;

            (b)   limitations to terminate employment agreements, including
                  providing for severance payments;

<Page>

                                     -59-


            (c)   obligations of the Material Messer Companies to make specific
                  investments or to guarantee a certain number of employees, all
                  at one or several specific sites.

            Except as set forth in EXHIBIT 5.12(c), at the Material Messer
            Companies there have not been any strikes or unrests of employees
            since January 1, 2000. Unless otherwise specified, the guarantees
            contained in this Section 5.12 are made as of the date hereof.

      5.13  EMPLOYEE BENEFIT PLANS. Certain of the Material Messer Companies, in
            addition to subscribing their employees to mandatory national
            social security schemes, do have employee plans in place providing
            for additional retirement-, death-, disability- or health benefits
            (collectively the "PLANS"). These employee benefit plans may have
            been adopted either by voluntary company decision or because of
            existing labor law obligations in this respect. All Plans providing
            for retirement benefits in Consolidated Companies are reflected in
            EXHIBIT 5.13. All Plans as well as the plans listed in EXHIBIT
            5.10(a)(i) and (ii) have been established and administered in
            compliance with Applicable Law and regulations unless otherwise
            disclosed in EXHIBIT 5.13. All company contributions and payments of
            the Material Messer Companies both under mandatory social security
            schemes as well as under additional retirement, death, disability,
            or health benefit plans have been paid when due or the amounts due
            have been reserved in the 1999 accounts of a respective Material
            Messer Company or in the Financial Statements 1999. All Social
            Security Payments not having been made by the Financial Date and
            relating to the time until December 31, 1999 have been fully
            provided for in the Financial Statements 1999 or in the individual
            financial statements of the Material Messer Companies as of December
            31, 1999. Each U.S. employee benefit plan which is intended to be
            qualified within the meaning of Section 401(a) of the United States
            Internal Revenue Code of 1986, as amended, is so qualified and has
            received to the required extent a favorable determination letter
            from the Internal Revenue Service as to its qualification and
            nothing has occurred, whether by action or failure to act that could
            reasonably be expected to cause the loss of such qualification. For
            defined contribution plans all contributions due as of December 31,
            1999 to external providers have either been paid in full or have

<Page>

                                      -60-


            been accrued on the Financial Statements 1999. For defined benefit
            plans accrued liabilities and annual cost have been determined on
            basis of local valuation methods that are in line with, or are not
            materially different from, IAS 19. With respect to each Plan that is
            subject to ERISA, no "reportable event" (as defined in Section 4043
            of ERISA) that could have adverse financial consequences, no
            "prohibited transaction" (as defined in Section 406 of ERISA), and
            no "accumulated funding deficiency" has occurred. None of the
            Material Messer Companies and none of the ERISA Affiliates (as
            defined as entities treated as being a single employer together with
            any Material Messer Company under Section 414 of the Code) of the
            Material Messer Companies have incurred any liability under Title
            IV of ERISA.

      5.14  CAPITAL CONTRIBUTIONS/DISTRIBUTIONS. Except as set forth in EXHIBIT
            5.14, there are no outstanding payment obligations
            (EINLAGEVERPFLICHTUNGEN) with respect to the Messer Capital Stock
            for which any Messer Group Company (except for the Participations)
            is liable, and there have been no (direct or indirect) hidden or
            open repayments of the Messer Capital Stock (including the Hoechst
            MGG Share) to the shareholders of the Messer Group Companies which
            lead to a corresponding repayment obligation of the recipient or
            violate Applicable Law. As of the date hereof and except as
            disclosed in EXHIBIT 5.14 or as expressly agreed in the Corporate
            Documents, no Material Messer Company is obliged to make further
            capital contributions (NACHSCHUSSVERPFLICHTUNGEN) to any Messer
            Group Companies on the basis of corporate resolutions.

      5.15  COMPLIANCE WITH LAWS; PERMITS; CERTAIN OPERATIONS. Except as
            disclosed in EXHIBIT 5.15, each of the Material Messer Companies
            holds all the Permits necessary and material to the conduct of the
            Gas Business, and all such Permits are in full force and effect.
            Except as stated in EXHIBIT 5.15, as of the date hereof, the
            Material Messer Companies have not received any written and binding
            notice from any public authority revoking, canceling, rescinding,
            materially modifying or refusing to renew any Permit material to the
            conducting of the Gas Business or providing written notice of
            violations material to the conducting of the Gas Business under any
            law. The Material Messer Companies are in all material respects in
            compliance with the Permits and Applicable Law and the

<Page>

                                     -61-


            requirements of the Permits and have filed all declarations and
            reports required by Applicable Law, directives and material
            regulations.

      5.16  INSURANCE. The Material Messer Companies except the companies
            located in Egypt have retained insurance coverage as customary in
            the industrial gases industry in the respective country. All
            insurance premiums have been paid by the Material Messer Companies
            when due. No party has given a notice to terminate a material
            insurance contract. To Hoechst's best knowledge no Material Messer
            Company is in default with any material obligations (including
            OBLIEGENHEITEN) under such insurance contracts.

      5.17  OTHER LIABILITIES. Other than guarantees referred to in Section 5.10
            (a)(iv), and (i) to be disclosed in the Exhibits to this Section 5
            or (ii) involving not more than EUR 2,500,000, there are no
            liabilities exceeding an amount of EUR 25,000,000 in the aggregate
            of any Material Messer Company for circumstances attributable to the
            time prior to the date hereof which are (as to the internal
            relationship) solely stemming from any obligations of the Messer
            Group Companies other than the Material Messer Companies.

      5.18  FINANCE LEASES. As of the Financial Date, the total amount
            (determined on a basis consistent with the Financial Statements
            1999) of all Finance Leases of all Consolidated Messer Companies,
            which individually have an accounted value in excess of EUR
            2,500,000, did not exceed DM 275,560,000 (without taking into
            account the so-called "Duisburg Lease".

      5.19  CONSOLIDATED DEBT AT YEAR END. The Total Debt of the Company and its
            Consolidated Subsidiaries will not, as at December 31, 2000, be more
            than DM 3,700,000,000.

            For this purpose, "Total Debt" means the sum of (without double
            counting):

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                                      -62-


            (a)   corporate debt;

            (b)   liabilities due to stockholders;

            (c)   accrued interest;

            (d)   guarantees; and

            (e)   share capital not yet fully paid in and extended liability,

            in each case as shown in the consolidated audited IAS financial
            statements of the Company and for the business year ending on
            December 31, 2000, including the notes thereto, adjusted by:

            (f)   adding (to the extent not otherwise included) the total amount
                  actually or contingently due under factoring arrangements;

            (g)   deducting the total amount of all actual or contingent
                  liabilities of the Company or any of its Consolidated
                  Subsidiaries in respect of Messer Singapore and Singapore
                  Syngas to the extent otherwise reflected in the above, but
                  adding an amount equal to DM 180,000,000, reduced by the
                  difference between the aggregate expenses incurred by the
                  Company relating to Messer Singapore and Singapore Syngas
                  between September 1, 2000 and December 31,2000 and the
                  aggregate amounts to be paid by Hoechst on the Closing Date
                  under paragraph 4.5(h) pursuant to the Singapore Separation
                  Agreement;

            (h)   deducting the amounts to be paid by Hoechst to MGG on the
                  Closing Date pursuant to Section 4.17(m) and pursuant to
                  Section 4.5 of the Singapore Separation Agreement in each case
                  related to expenses incurred by MGG between September 1, 2000
                  and December 31, 2000;

            (i)   deducting the total amount included under paragraph (d) above,
                  to the extent that they are covered by the specific indemnity
                  from Hoechst under Section 4.17

<Page>

                                      -63-


            Items (a) to (e) above will be prepared applying the same
            definitions, accounting and disclosure principles as for the
            preparation of the Financial Statements 1999.

            It is agreed that a loan of the International Finance Corporation in
            the amount of US$ 51,000,000 extended to Messer Trinidad and Tobago
            Ltd. shall not be taken into account when determining the amount of
            the Total Debt; PROVIDED that there is an equal amount of cash on
            hand and reflected in the Financial Statements 2000.

            The guarantee contained in this Section 5.19 shall not be subject to
            the De Minimis Amount, the Threshold and the Guarantee Cap. In the
            event of a breach of the guarantee contained in this Section 5.19,
            any Loss to be indemnified hereunder shall be reduced by the debt
            amounts under (e) above, except in a situation in which, and then
            only to the extent that, (i) the payment to the relevant company
            concerned must be made to the capital reserves (i.e., without
            consideration) and exceeds the direct or indirect pro rata
            shareholding of MGG or (ii) the payment is made to a company which
            at the time of the payment is Insolvent and the paid amounts cannot
            reasonably be expected to be recovered.

      5.20  SPECIAL REP. No Messer Group Company, Aventis China Company or ACIC
            is currently engaged in, and has not within the prior three years
            engaged in, any activity described in EXHIBIT E. The foregoing
            shall not relate to the activities of MGG as the former owner of any
            businesses to the extent no longer directly or indirectly owned or
            operated by MGG (including the cutting and welding business).

6.    INDEMNIFICATION BY HOECHST

      6.1   GENERAL. If any of the guarantees in Section 5 is untrue or
            incorrect when made, Hoechst may take all such actions as shall be
            necessary or desirable to cure the breach, including any Losses
            resulting therefrom. If and to the extent Hoechst fails to cure the
            breach within a period of three (3) months after notification by
            Investor NewCo, solely Investor NewCo

<Page>

                                      -64-


            shall be entitled to raise claims and it shall only be entitled to
            claim damages in money from Hoechst according to this Section 6.

      6.2   LOSS. Investor NewCo shall only be entitled to claim damages
            pursuant to the provisions of this Section 6 for liabilities,
            losses, damages, claims, external costs and expenses, interest,
            awards, judgments and penalties (including, without limitation,
            attorneys' and consultants' fees and expenses) ("LOSS") actually and
            directly suffered or incurred by Investor NewCo or any of the Messer
            Group Companies, as applicable, arising out of or resulting from the
            breach of any guarantee made by Hoechst in Section 5. Any such Loss
            shall only include future loss of profits ("ENTGANGENER GEWINN") to
            the extent that ordinary course income (on an after tax basis) of
            any Messer Group Company which has suffered such Loss is reduced
            thereby (thus, e.g., any Loss resulting from a breach of Section 5.5
            would not include any loss of profits).

      6.3   HOECHST DAMAGE QUOTA, DE MINIMIS AMOUNT, THRESHOLD AND CAP. Investor
            NewCo's claims for damages shall be based upon the principle that
            (i) Hoechst shall only be liable under this Section 6 for a certain
            quota of any incurred Losses (the "HOECHST DAMAGE QUOTA"), which
            quota shall be based upon the rules explained under (a) below; that
            (ii) except for specified cases, a de minimis amount and a threshold
            must be observed as explained under (b) below and that (iii)
            additionally a cap shall apply as explained under (c) below.

            (a)   The Hoechst Damage Quota shall be computed as follows:

                  (i)   If a Fully Owned Messer Group Company incurs Losses, the
                        Hoechst Damage Quota shall be sixty-six and two-thirds
                        percent (66 2/3 %) of all Losses incurred by such Fully
                        Owned Messer Group Company (the "66% PRINCIPLE");

                  (ii)  Subject to (iii) and (iv) below, if a Non-Fully Owned
                        Messer Group Company incurs any Losses, the Hoechst
                        Damage Quota shall be equal to the pro rata amount of
                        the shareholding directly or indirectly transferred to
                        Investor NewCo hereunder (the "PRO RATA SHAREHOLDING

<Page>

                                     -65-


                        PRINCIPLE", together with the 66% Principle the "PRO
                        RATA PRINCIPLE"); For example: In case of a 51% owned
                        Messer Group Company, the Hoechst Damage Quota amounts
                        to thirty-four percent (34%) (66 2/3 x 51%).

                  (iii) The Pro Rata Principle shall not apply with respect to
                        any Loss which is directly incurred by Investor NewCo
                        (through Hoechst NewCo II) and which is not incurred at
                        the same time by any of the Messer Group Companies
                        (i.e., any Loss related to the Hoechst MGG Share
                        itself), for such Losses the Hoechst Damage Quota shall
                        amount to 100%, subject to the other limitations
                        contained in this Section 6 (the "PRO RATA EXCEPTION
                        PRINCIPLE").

                  (iv)  For the avoidance of doubt: There shall be no double
                        counting of Losses so that, for example, the reduced
                        value of a Subsidiary or reduced dividend flows
                        resulting from a Loss and incurred by the respective
                        shareholding company shall not be indemnifiable
                        hereunder. If, however, such Loss as a result of a
                        breach of a guarantee or other indemnity obligation is
                        also directly suffered or incurred by a Messer Group
                        Company in which Investor NewCo indirectly holds a
                        higher shareholding, then such higher shareholding shall
                        be relevant for the application of the Pro Rata
                        Principle and only the Loss suffered by this company
                        shall be relevant.

            (b)   Hoechst shall in any event only be liable under this Section
                  6, if the individual claim or series of claims (SERIENSCHADEN)
                  recoverable under this Section 6 exceeds EUR 250,000 (in
                  words: Euro two hundred fifty thousand) in case of a breach of
                  a guarantee other than the guarantees contained in Section 5.5
                  and EUR 1,000,000 (in words: Euro one million) in case of a
                  breach of a guarantee contained in Section 5.5 (each amount a
                  "DE MINIMIS AMOUNT") and the aggregate of all individual
                  claims above the De Minimis Amount exceeds EUR 10,000,000 (in
                  words: Euro ten million) (the "THRESHOLD"), in which case the
                  entire amount except for any De Minimis Amounts is payable;
                  PRO-

<Page>

                                      -66-


                  VIDED FURTHER that in case of a breach of any of the
                  guarantees stated in Sections 5.1 through 5.4 Investor NewCo
                  shall be entitled to receive the full amount of any such
                  recoverable individual claim or series of claims irrespective
                  of the amount up to the Total Compensation;

            (c)   In no event shall Hoechst's aggregate liability under this
                  Section 6 (in connection with Section 5) exceed the amount of
                  twenty percent (20%) of the Total Compensation (the
                  "GUARANTEE CAP"); PROVIDED that the Guarantee Cap shall not
                  apply in case of a breach of any of the guarantees stated in
                  Sections 5.1 through 5.4. For the avoidance of doubt: In no
                  event shall Hoechst be obliged to make indemnifications under
                  Sections 5, 6 and 9 in excess of the aggregate amount of the
                  Total Compensation. It is understood that if the Total
                  Compensation is reduced as a result of a non consummation of
                  the China SPA, any payments made by Hoechst exceeding the
                  reduced amount of the Total Compensation shall be promptly
                  refunded to Hoechst by Investor NewCo.

      6.4   THIRD-PARTY-CLAIMS. If Investor NewCo determines to seek remedies
            under this Section 6 (in connection with Section 5 above), with
            respect to claims resulting from the assertion of liability by
            third parties, Investor NewCo shall give written notice to Hoechst
            as soon as possible but not later than fifteen (15) Business Days
            after Investor NewCo or the Messer Group Companies affected become
            aware of any such claim. The parties shall then mutually agree
            whether and which steps to take to defend themselves against such
            claims. If requested by Hoechst, Investor NewCo or any of the Messer
            Group Companies concerned shall defend itself against such claims as
            directed by Hoechst; PROVIDED that Hoechst agrees to bear the costs
            connected therewith, whereby general administration costs of
            Investor NewCo or any of the Messer Group Companies concerned shall
            always be borne by Investor NewCo.

      6.5   WAIVER OF CLAIMS. Any claims of Investor NewCo pursuant to this
            Agreement shall be deemed waived by Investor NewCo, if Investor
            NewCo has not submitted to Hoechst within twenty (20) Business Days

<Page>

                                      -67-


            after discovery of the relevant facts by Investor NewCo or any of
            the Messer Group Companies affected justifying a damage claim
            against Hoechst hereunder a written notice specifying in reasonable
            detail the facts upon which an alleged or possible claim is based,
            if and to the extent Hoechst is unable to mitigate its damage as a
            result thereof.

      6.6   LIMITATION. Any claims pursuant to this Section 6 (in connection
            with a breach of guarantees other than those stated in Sections 5.1
            through 5.4) shall be time-barred in the case of any claims made by
            Investor NewCo after June 30, 2002 and in the case of any claims
            made by MGG or any MGG Designee under the China SPA 18 months after
            the China Closing Date (the "CHINA LIMITATION DATE"), unless
            Investor NewCo or MGG or any MGG Designee, as the case may be, (i)
            has submitted to Hoechst a notice in accordance with Section 6.5
            hereof at the latest on or before June 30, 2002 or the China
            Limitation Date, and (ii) has initiated arbitration proceedings in
            accordance with Section 12.16 within a period of three (3) months
            after receipt of the notice by Hoechst referred to in sub-paragraph
            (i) above if such claims are not recognized by Hoechst until such
            date. Claims for a breach of the guarantees stated in Sections 5.1
            through 5.4 shall be time-barred after the tenth (10th) anniversary
            of the Closing Date in the case of claims made by Investor NewCo
            hereunder, and after the tenth (10th) anniversary of the China
            Closing Date, in the case of claims made by MGG or any MGG Designee
            under the China SPA.

      6.7   MITIGATION. Investor NewCo shall undertake reasonable efforts to
            ensure that all reasonable steps are taken which are necessary to
            avoid a liability of Hoechst under this Agreement being incurred and
            to mitigate the amount of any such liability. When calculating the
            amount of any such liability, all present and future advantages
            (including tax advantages) in connection with the subject matter
            shall be taken into account.

            To the extent the registered share capital of a Messer Group Company
            has not been fully paid in or has been repaid in violation of
            Sections 5.2 or 5.14 prior to the date hereof (the "PRE-TRANSACTION
            DISTRIBUTED CAPITAL SITUATION"), the following shall apply without
            limiting the foregoing:

<Page>

                                     -68-


            (a)   To the extent that a Pre-Transaction Distributed Capital
                  Situation constitutes a Loss to a Messer Group Company,
                  Hoechst shall indemnify Investor NewCo in accordance with the
                  terms set forth in this Section 6.

            (b)   In case of a Pre-Transaction Distributed Capital Situation at
                  MGG and independent of any Loss suffered by Investor NewCo,
                  Investor NewCo and MIG shall to the extent (i) permitted under
                  then Applicable Law and under any contracts MGG and its
                  Affiliates are a party to, and (ii) possible without any
                  disadvantages of MGG which are not compensated in advance by
                  Hoechst, use reasonable efforts that the stated capital of MGG
                  will be reduced, if necessary, such that the full amount of
                  the capital previously required to be paid to MGG can be
                  redistributed to Investor NewCo. Any such distribution to
                  Investor NewCo shall be forwarded to Hoechst when received net
                  of taxes up to the amount received by Investor NewCo or any
                  other intermediate holding company under (a). Any refundable
                  withholding tax will be balanced among the parties upon
                  availability of such refunds. MIG and Investor NewCo shall use
                  reasonable efforts that any such capital reductions will be
                  made and any such refunds will be available as soon as
                  possible.

                  In case of a Pre-Transaction Distributed Capital Situation at
                  any Messer Group Company other than MGG and independent of any
                  Loss suffered by Investor NewCo, Investor NewCo shall, to the
                  extent (i) permitted under then Applicable law and under any
                  contracts the respective Messer Group Company, MGG or their
                  Affiliates are a party to, and (ii) possible without any
                  disadvantage of MGG, the respective Messer Group Company and
                  their Affiliates which are not compensated in advance by
                  Hoechst, use reasonable efforts, that such other MGG Group
                  Company's stated capital will be reduced, if necessary, such
                  that the full amount of the capital previously required to be
                  paid to such MGG Group Company can be redistributed to the
                  respective shareholding company or companies. To the extent
                  any such redistribution(s) diminishes the Loss of Investor
                  NewCo, Investor NewCo shall

<Page>

                                      -69-


                  refund to Hoechst any indemnification earlier granted to it by
                  Hoechst under (a). Investor NewCo shall use reasonable efforts
                  that any such funds will be available as soon as possible at
                  the Messer Group Company or Companies having suffered the
                  Loss.

            (c)   Upon a disposal of any shares in MGG (other than through a
                  public offering) by Investor NewCo, Investor NewCo is
                  obligated to assign any rights under this Section 6.7 to a
                  purchaser or successor and such purchaser or successor shall
                  assume the obligations of Investor NewCo (on a pro rata basis,
                  if applicable) under this Section 6.7. The rights and
                  obligations of Investor NewCo under the foregoing shall
                  terminate on June 30, 2002.

            (d)   If a mitigation of any damage in accordance with the foregoing
                  is not achievable within a reasonable time period, the parties
                  shall consult in good faith to find appropriate alternative
                  solutions.

<Page>

                                      -70-


      6.8   INDEMNIFICATION OF HOECHST. To the extent Hoechst is not liable
            (including as a result of periods of limitation) under this
            Agreement but Hoechst or any of its Affiliates are liable for a
            liability of any of the Material Messer Companies on the basis of a
            "piercing-the-corporate-veil" concept, Investor NewCo shall
            indemnify and hold harmless Hoechst and its Affiliates from Losses
            resulting therefrom, unless the liability of Hoechst and its
            Affiliates is based on a specific action or omission of the parties
            to be indemnified hereunder; PROVIDED that the claim for
            indemnification or series of claims (SERIENSCHADEN) exceeds in each
            case EUR 250,000; and PROVIDED FURTHER that Investor NewCo shall not
            be liable under this Section 6.8 in excess of an amount equal to the
            Guarantee Cap. Any liability of Investor NewCo under this Section
            6.8 shall be subject to a period of limitation ending December 31,
            2003; PROVIDED that with respect to claims relating to taxes the
            period of limitation shall not end before the relevant period set
            forth in Section 8.8(c) shall have expired. Sections 6.4, 6.5 and
            6.7 shall apply MUTATIS MUTANDIS. For the avoidance of doubt: Any
            statutory claims of Aventis or its Affiliates shall not be limited
            by the foregoing.

      6.9   SOLE REMEDY. Investor NewCo hereby acknowledges and agrees that its
            sole and exclusive remedy with respect to any and all Losses
            relating to the subject matter of this Agreement shall be pursuant
            to the applicable indemnification provisions of the Combination
            Documents. Any breach of a representation under Section 5 is
            exclusively being indemnified pursuant to this Section 6, PROVIDED
            that the indemnification for Environmental Contamination shall be
            exclusively governed by Section 9 and that the indemnification for
            Taxes shall be exclusively governed by Section 9 and that the
            indemnification for Covered Guarantees shall be exclusively governed
            by Section 4.17. In furtherance of the foregoing, Investor NewCo
            hereby waives to the fullest extent permitted under applicable law
            any and all other statutory or contractual claims relating to the
            subject matter of this Agreement not covered by this Section 6 (in
            connection with Section 5), Sections 8 and 9. As amplification and
            without limitation any claims based on POSITIVE VERTRAGSVERLETZUNG
            and CULPA IN CONTRAHENDO and any claims for cancellation (WANDLUNG)
            and rescission (RUCKABWICKLUNG) or challenge (ANFECHTUNGSRECHTE) of
            this Agreement, except for the challenge because of malicious deceit
            (ANFECHTUNG WEGEN

<Page>

                                      -71-


            ARGLISTIGER TAUSCHUNG), are expressly excluded; PROVIDED that any
            claims for fulfillment of this Agreement (ERFULLUNGSANSPRUCHE) which
            are not expressly and specifically the subject matter of a guarantee
            under Section 5 of this Agreement, and claims for breach of
            covenants and other obligations under this Agreement and the rights
            under Section 10.1 are not affected by this Section 6.9, regardless
            of whether raised by Investor NewCo or any other party under this
            Agreement or any other Combination Documents contemplated to be
            executed by this Agreement.

      6.10  EXCLUSION OF LIABILITY. Any liability of Hoechst to indemnify
            Investor NewCo for a breach of representations pursuant to this
            Section 6 or pursuant to Sections 8 and 9 is excluded if and up to
            the amount the underlying circumstances (i) are provided for in the
            Financial Statements 1999 or described in EXHIBIT 5.5 and have not
            been used up as of the Closing Date, (ii) are provided for in the
            individual audited financial statements of the relevant Material
            Messer Company as of December 31, 1999 or described in EXHIBIT 5.5
            and have not been used up as of the Closing Date, or (iii) are the
            subject of valid and collectable claims for corresponding
            performances of third parties, in particular by insurances (the
            validity risk and the insolvency risk and all Losses associated with
            the collection of such claims regarding the third party shall be
            borne by Hoechst).

      6.11  ORAL STATEMENTS. Hoechst is not liable for the correctness of any
            oral or written statements made by managing directors, employees or
            consultants of Hoechst, its Affiliates, MIG and the Messer Group
            Companies in particular during the Due Diligence, and the knowledge
            of these persons cannot, subject to Section 6.12 and unless
            otherwise provided in this Agreement, be attributed to Hoechst. This
            Section 6.11 does not apply to any covenants or other obligations
            not related to a guarantee under Section 5 of this Agreement.

            Without limiting the foregoing, Hoechst or Hoechst NewCo III are in
            particular not liable for the correctness and completeness of any
            oral or written information provided to KPMG in connection with the
            Due Diligence.

<Page>

                                      -72-


      6.12  KNOWLEDGEABLE PERSONS. To the extent Hoechst has made a
            representation based on "Hoechst's best knowledge", Hoechst can only
            be held liable if any of the following persons were aware of the
            incorrectness of the representation or were unaware of it due to
            their own negligence: Mr. Waesche, Dr. Schmieder, Dr. Schottler, Mr.
            Dornedden, Mr. Stowasser, Mr. von Storm, Mr. Allcock and Mr. Doerr.

      6.13  EXCLUSION OF SECTIONS 439, 460, 464. For the avoidance of doubt:
            Sections 439, 460, 464 of the German Civil Code (BGB) are excluded.

      6.14  AVENTIS CHINA COMPANIES. In case any of the guarantees in Section 5
            made with respect to ACIC, New China Holding (as defined in the
            China SPA), any of the ACIC Gas Companies (as defined in the China
            SPA and being identical with the Aventis China Companies), any other
            Chinese company sold to MGG directly or indirectly in connection
            with any sale and transfer mechanism or the MCCO Assets is breached,
            Sections 6.1 through 6.13 shall apply; provided that the
            66%-Principle shall not apply but the Pro Rata Shareholding
            Principle shall apply and that any indemnifications shall be made by
            Hoechst to MGG and FURTHER PROVIDED that the guarantees given in
            Sections 5.1 through 5.4 are not made as of the Closing Date but as
            of the China Closing Date and thus may not affect the closing
            condition set forth in Section 3.1(g).

      6.15  AUDIT REVIEW. With respect to a potential breach of the guarantee
            stated in Section 5.19, MGG will cause that KPMG shall audit the
            Financial Statements 2000 as promptly as possible and such audited
            Financial Statements 2000 shall be submitted to Hoechst and Investor
            NewCo promptly thereafter. Hoechst, and Investor NewCo, shall each
            have the right to appoint an auditor who shall then have the right
            to review such Financial Statements 2000. For this purpose Hoechst
            and Investor NewCo and their auditors shall have access to the books
            and records used by MGG and the Consolidated Messer Companies in the
            preparation of the Financial Statements 2000 and shall have
            reasonable access to the working papers of KPMG prepared in
            connection with the audit. In the event Hoechst or Investor NewCo
            does not agree with the delivered Financial Statements 2000 to the
            extent relevant for Section 5.19, it shall

<Page>

                                      -73-


            deliver to the other a written objection (the "OBJECTION NOTICE")
            within a period of five (5) weeks after receipt of the Financial
            Statements 2000. In the event that no Objection Notices are
            delivered within said period, the Financial Statements 2000 shall
            become final and binding for purposes of determining a breach of
            Section 5.19, without prejudice to any other guarantees given
            pursuant to Section 5. In the event that an Objection Notice is
            delivered, the parties shall use best efforts to resolve such
            objection. To the extent that the parties then agree on the items of
            the Financial Statements 2000 relevant for Section 5.19, such
            agreement shall become final and binding for the parties.

            If and to the extent that Investor NewCo and Hoechst cannot agree on
            the items of the Financial Statements 2000 relevant for Section
            5.19 within four (4) weeks following the delivery of an Objection
            Notice, the disputed items shall be decided with binding effect for
            the Parties by BDO or any other internationally recognized auditing
            firm mutually agreed by Investor NewCo and Hoechst (the "FIRM") in
            accordance with Section 317 German Civil Code (BGB). Before
            rendering its decision, the Firm must grant Hoechst and Investor
            NewCo an opportunity to present their views. The Firm has to submit
            its decisions with reasons in writing to Investor NewCo and Hoechst
            and shall be instructed to do so within five (5) weeks of the
            commencement of such proceedings. With respect to the costs of the
            proceeding, Section 91 et seq. of the German Code of Civil Procedure
            (ZPO) shall apply. Such costs shall be shared by Investor NewCo and
            Hoechst.

      6.16  SPECIFIC REMEDY FOR SECTION 5.20. Hoechst shall indemnify Investor
            NewCo, subject to the Pro Rata Principle and the Guarantee Cap and
            subject to the other provisions of this Section 6 except for the De
            Minimis Amount and the Threshold, for any Losses arising out of or
            resulting from any breach of the guarantee set forth in Section
            5.20, PROVIDED that the matter giving rise to such breach was within
            Hoechst's best knowledge on the date of the execution of this
            Agreement AND such matter was not disclosed to Investor NewCo prior
            to the Closing Date.

<Page>

                                      -74-


7.    REPRESENTATIONS AND WARRANTIES OF INVESTOR NEWCO

      7.1   REPRESENTATIONS. Investor NewCo represents and warrants in the form
            of an independent guarantee to Hoechst that as of the date hereof
            and the Closing Date:

            (a)   Investor NewCo is duly incorporated and validly existing under
                  the laws of the jurisdiction of incorporation and has all
                  requisite corporate power and authority to own assets and to
                  carry out business. The execution, delivery and performance by
                  Investor NewCo of this Agreement does not, and the
                  consummation of the transactions contemplated herein will not
                  conflict with or result in a breach of the terms, conditions
                  or provisions of or constitute a default under its articles of
                  incorporation or by-laws under which it is bound or to which
                  it is subject.

            (b)   Investor NewCo has delivered to Hoechst prior to the signing
                  of this Agreement a complete and correct copy of three
                  commitment letters of Allianz AG, Allianz Capital Partners
                  GmbH and of GS Capital Partners 2000, L.P. (the "FUND") duly
                  executed and not terminated as of the date of signing of this
                  Agreement.

            (c)   Immediately prior to the Closing Date, the sole shareholders
                  of Investor NewCo are the GS Funds and Allianz Capital
                  Partners GmbH subject to a transfer of up to 17% of the shares
                  to a third party until the Closing Date; PROVIDED that such
                  transfer shall not have a negative impact on the assessment of
                  the transactions contemplated hereunder under applicable
                  merger control laws or procedures.

      7.2   INDEMNIFICATION. In the event that Investor NewCo is in breach of
            any representation under Section 7.1 above, it shall indemnify
            Hoechst for any Loss, including lost profits, as a result thereof.
            Claims for a breach of the guarantees stated in this Section 7 shall
            be time-barred after the tenth (10th) anniversary of the Closing
            Date.

<Page>

                                      -75-


8.    TAX MATTERS

      8.1   TAX INDEMNITIES.

            (a)   Hoechst agrees to indemnify, subject to the Pro Rata Principle
                  and the Pro Rata Exception Principle but not subject to any of
                  the limitations and restrictions set forth in Sections 5 and
                  6, Investor NewCo against

                  (A)   all Taxes which have not been paid by any Tax
                        Indemnification Company by the Financial Date although
                        having been due by this date or if not due by this date
                        would have been due (based on past experience and local
                        standards) if a tax return that has not been filed when
                        due had been filed when due,

                  (B)   any German real estate transfer tax (GRUNDERWERBSTEUER)
                        resulting from (i) the transfer of the shares in
                        Hoechst NewCo II, which MIG receives as consideration
                        for the contribution of its interest in MGG to Hoechst
                        NewCo II, by MIG to Investor NewCo, PROVIDED that any
                        such contribution shall be made on or before December
                        31, 2001 and (ii) the transfer of the Hoechst NewCo III
                        Shares from Hoechst to Investor NewCo in case of an
                        exercise of the Counter-Call Option pursuant to Section
                        2.9.

                  (C)   all Taxes payable by a Tax Indemnification Company for
                        any taxable period or portion thereof that ends on or
                        before December 31, 1999,

                  (D)   all Taxes resulting from (x) the write up of the tax
                        base or (y) any taxable capital gain, both to the extent
                        related to the shares in Messer Netherlands B.V. or the
                        shares formerly held directly by MGG in Messer UK Ltd.,
                        Messer France S.A. and Messer Italy S.p.A., both (x) and
                        (y) to the extent that these Taxes are resulting from
                        the mandatory reversal of previous write-downs
                        (WERTAUFHOLUNGSGEBOT); PROVIDED that (x) and (y) only
                        apply to the

<Page>

                                      -76-


                        extent related to circumstances occurred in, and
                        attributable to, the time period before the Financial
                        Date and (x) and (y) shall not apply if a reversal of a
                        write-down is already indemnifiable under lit. C above,
                        and

                  (E)   all Losses resulting from any shortfall between the
                        realized amounts of refund claims against tax
                        authorities, which are scheduled to be received after
                        the Financial Date and the value at which such claims
                        are accounted for in the Financial Statements 1999;

                  PROVIDED, HOWEVER, that no indemnity shall be provided under
                  this Agreement for any Taxes (i) if such Taxes are reflected
                  in the Financial Statements 1999 or the financial statements
                  of the individual Tax Indemnification Company as of December
                  31, 1999, or have been paid on or before the Financial Date,
                  (ii) if and to the extent Investor NewCo does not fully comply
                  with its obligations under this Section 8 and Hoechst is
                  unable to mitigate its damages as a result thereof, (iii) if
                  such Taxes are resulting from any transaction undertaken or
                  caused by Investor NewCo, on or after the Closing Date except
                  for transactions contemplated by this Agreement and except as
                  for Taxes referred to under lit. B and D above, or (iv) to the
                  extent that such tax liability under lit. C above is based on
                  a reallocation of earnings or losses and such reallocation
                  will cause a tax reduction in a corresponding amount after
                  December 31, 1999; PROVIDED that the net present value of such
                  future tax reduction shall be calculated on the basis of a six
                  percent (6%) annual interest rate. No indemnity is provided
                  for any Tax payments resulting from (i) the Write-Downs
                  described in EXHIBIT 4.6, and (ii) the consummation of the
                  Foreign Sub Contribution Agreement.

            (b)   Payment by Hoechst of any indemnification due under this
                  Section 8.1 shall be made within ten days following written
                  notice by Investor NewCo, as the case may be, that payment of
                  such amounts to the appropriate Tax Authority is due, PROVIDED
                  that Hoechst shall not be required to make any payment earlier
                  than two (2) days before such Taxes are due to the appropriate
                  Tax

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                                      -77-


                  Authority or in case of losses from a shortfall or a refund
                  claim has not been paid by the Tax Authority when it was
                  scheduled to be due. In the case of any Tax that is contested
                  in accordance with the provisions of Section 8.6, payment of
                  such Tax to the appropriate Tax Authority will be considered
                  due no earlier than the date a final determination to such
                  effect is made by the appropriate Tax Authority or a court of
                  proper jurisdiction; PROVIDED that the competent tax authority
                  has granted relief from paying the assessed Tax until such Tax
                  becomes final and binding. If this is not the case, Hoechst
                  shall make a respective advance indemnification payment to
                  Investor NewCo, and if the final amount of Taxes to be paid is
                  lower than assumed when making the advance indemnification
                  payment by Hoechst, then the appropriate difference plus
                  interest, if any, shall be reimbursed by Investor NewCo, as
                  the case may be, including all interest thereon received or
                  otherwise credited by a Tax Authority.

            (c)   For purposes of this Agreement, (i) subject to Section 8.7 in
                  the case of any Tax based upon or measured by gross or net
                  income or receipts, net worth, capital or franchises, or any
                  Tax imposed in connection with any sale or other transfer of
                  property, that is payable for a period that begins before
                  December 31, 1999 and that ends after December 31, 1999, the
                  portion of any such Tax that is allocable to the portion of
                  the period ending on December 31, 1999 shall be deemed equal
                  to the amount that would be payable if the taxable period
                  ended on December 31, 1999, and (ii) in the case of any other
                  Tax that is payable for a period that begins before December
                  31, 1999 and that ends after December 31, 1999, the portion of
                  any such Tax that is allocable to the portion of the period
                  ending on December 31, 1999 shall be deemed to equal the
                  amount of Taxes for the entire period multiplied by a fraction
                  in which the numerator is the number of days in the portion
                  of the period ending on December 31, 1999 and the denominator
                  is the number of days in the entire period.

      8.2   REFUNDS AND BENEFITS.

<Page>

                                      -78-

            (a)   Investor NewCo shall promptly pay to Hoechst in accordance
                  with the Pro Rata Principle any Tax refund, credit or similar
                  benefits (including any interest paid or credited with respect
                  thereto) realized or received by any Tax Indemnification
                  Company or under a tax unity with respect to a Tax
                  Indemnification Company by any of their Affiliates and not
                  accounted for as an asset in the individual financial
                  statements of the respective Tax Indemnification Company as of
                  December 31, 1999,

                  (i)   relating to the Tax Indemnification Companies for
                        taxable periods or portions thereof ending on or before
                        December 31, 1999; or

                  (ii)  attributable to an amount paid by Hoechst under Section
                        8 hereof.

                  Investor NewCo shall in general and, if Hoechst so requests at
                  Hoechst's expense -- pro rata in accordance with the Pro Rata
                  Principle-, cause the Tax Indemnification Companies to file
                  for and obtain any Tax Refund, credits or similar benefits to
                  which Hoechst may be entitled under this Section 8.2 and if
                  reasonable to prosecute such claims. Investor NewCo shall
                  permit Hoechst to procure and control (at Hoechst's expense)
                  the prosecution of any such claim, and shall cause any of its
                  Affiliates or the Tax Indemnification Companies to authorize
                  by appropriate power of attorney to such persons as Hoechst
                  shall designate to represent such entity with respect to such
                  claim. In case a tax refund will be realized as a result of
                  such claim, the amount of such refund shall be shared in
                  accordance with the Pro Rata Principle.

            (b)   A Tax benefit is to be considered as realized for purposes of
                  this Agreement at the time that it is reflected on a Tax
                  assessment of the relevant entity and has reduced a Tax
                  liability otherwise payable or received in cash or as some
                  other cash equivalent.

            (c)   If a payment is made by Hoechst in accordance with Section
                  8.1, and if in a subsequent taxable year a Tax benefit is
                  realized by Investor NewCo or the Tax Indemnification
                  Companies, or under

<Page>

                                      -79-


                  a tax unity with respect to a Tax Indemnification Company by
                  any other Affiliates of Investor NewCo that was not previously
                  taken into account to reduce an amount otherwise payable by
                  Hoechst under Section 8.1, Investor NewCo shall promptly pay
                  to Hoechst, at the time of realization, the amount of such Tax
                  benefit to the extent that such amount would have resulted in
                  a reduction in the obligations of Hoechst under Section 8.1 if
                  the Tax benefit had been obtained or known in the year of
                  Hoechst's payment. This provision shall apply, MUTATIS
                  MUTANDIS, in case of refunds, so that refunds might be set-off
                  against future additional taxes arising out of the event that
                  caused such refund.

            (d)   To verify that Investor NewCo has fulfilled its obligation
                  under this Section 8.2, it shall, after the statute of
                  limitations for all claims of Investor NewCo under Section 8.1
                  has run out with respect to all Consolidated Companies,
                  account for all received refunds or Tax credits or similar
                  benefits and proceeds from the release of provisions made for
                  Taxes. Any then outstanding amounts including default
                  interest, if applicable, payable by Investor NewCo to Hoechst
                  shall then promptly be paid; PROVIDED that Investor New Co is
                  entitled to set-off such claims against any outstanding
                  claims.

      8.3   PREPARATION OF TAX RETURNS. MGG shall prepare and file (or cause to
            be prepared and filed) all Tax Returns relating to the Tax
            Indemnification Companies that relate to any taxable period ending
            on or before December 31, 1999 or that include transactions for
            which indemnification is granted in Section 8.1. Tax Returns filed
            by a Tax Indemnification Company or by any Affiliates under a tax
            unity for a taxable year that includes December 31, 1999 shall be
            prepared on a basis consistent with those prepared for prior tax
            years (unless counsel for Investor NewCo, after consultation with
            counsel for Hoechst, determines that there is no reasonable basis in
            law therefor). With respect to any Tax Return required to be filed
            by Investor NewCo or a Tax Indemnification Company after December
            31, 1999, and as to which an amount of Tax is allocable to Hoechst
            under Section 8.1, or as to which a Tax refund, credit or other Tax
            benefit is allocable to Hoechst under Section 8.2, Investor NewCo

<Page>

                                      -80-


            shall provide Hoechst with a copy of such completed return, and a
            statement (with which Investor NewCo will make available supporting
            schedules and information) certifying the amount of Tax or Tax
            refund, credit or other benefit shown on such return that is
            allocable to Hoechst pursuant to Section 8.1 or 8.2 hereof, at least
            two (2) weeks prior to the due date (including any extension
            thereof) for the filing of such return. Investor NewCo agrees to
            consult with Hoechst and to attempt in good faith to resolve any
            issues arising as a result of the review of such return and
            statement by Hoechst.

      8.4   COOPERATION AND EXCHANGE OF INFORMATION. Hoechst and Investor NewCo
            will provide each other with such cooperation and information as
            either of them reasonably may request of the other (and Investor
            NewCo shall cause the Tax Indemnification Companies to provide such
            cooperation and information) in filing any Tax Return, amended
            return or claim for refund, determining any liability for Taxes or a
            right to a refund of Taxes or participating in or conducting any
            audit or other proceeding in respect of taxes relating to the Tax
            Indemnification Companies. Such cooperation and information shall
            include providing copies of relevant Tax Returns or portions
            thereof, together with accompanying schedules and related work
            papers and documents relating to rulings or other determinations by
            Tax Authorities. Hoechst and Investor NewCo shall make its employees
            available (and Investor NewCo shall cause the employees of the Tax
            Indemnification Companies to be available) on a mutually convenient
            basis to provide explanations of any documents or information
            provided hereunder. Investor NewCo will retain all returns,
            schedules and work papers and all material records or other
            documents in its possession relating to Tax matters of the Tax
            Indemnification Companies which are subject to indemnification under
            Section 8.1 or reimbursement until the later of (i) the expiration
            of the statute of limitations of the taxable periods to which such
            returns and other documents relate, without regard to extensions, or
            (ii) six years following the due date (without extension) for such
            returns. After such time, before Investor NewCo shall dispose of any
            such documents, Investor NewCo, shall, by 90 days, prior written
            notice to Hoechst, give Hoechst the opportunity (at such Hoechst's
            expense) to remove and retain all or any part

<Page>

                                      -81-


            of such documents as Hoechst may select. Investor NewCo is bound to
            ensure that Hoechst is given the opportunity to attend fiscal Tax
            audits of the Tax Indemnification Companies concerning matters which
            are subject to indemnification under Section 8.1 or reimbursement
            under Section 8.2. Any discussions and negotiations with the Tax
            Authorities relating to such matters shall be made after
            consultation with Hoechst and considering the interests of Hoechst,
            notwithstanding the statutory obligations of the management of the
            Tax Indemnification Company concerned.

      8.5   NOTIFICATION. After the Closing Date, Investor NewCo shall notify
            Hoechst within two weeks upon learning of but in any event prior to
            the commencement of any Tax audit or administrative or judicial
            proceeding that, if determined adversely to the Tax Indemnification
            Company concerned would possibly constitute grounds for
            indemnification under Section 8.1. Such notice shall be in writing
            and shall contain factual information (to the extent known to
            Investor NewCo) describing the asserted Tax liability in reasonable
            detail and shall include copies of any notice or other document
            received from any Tax Authority in respect of any such asserted Tax
            liability.

      8.6   CONTEST. Investor NewCo shall procure that each of the Tax
            Indemnification Companies shall contest any Tax assessments or
            accompanying acts of the Tax Authorities in accordance with prudent
            business standards as if no indemnification were granted hereunder
            (any such contest related to Tax assessments or accompanying acts of
            Tax Authorities, which might give rise to an indemnification under
            this Section 8, collectively a "CONTEST"). In case Investor NewCo
            and Hoechst disagree on the success chances of any such Contest and
            the Tax Indemnification Company concerned is therefore not prepared
            to initiate any judicial proceeding, such proceedings shall be
            initiated at the request and expense of Hoechst, which then may also
            direct, through counsel of its own choosing such proceeding to the
            extent involving any asserted liability with respect to which
            indemnity may be sought under Section 8.1, however, Investor NewCo
            may, at its own expenses, continue to participate in the Contest. If
            Hoechst elects to direct a Contest, then Hoechst shall, within three
            (3) weeks of receipt of Investor NewCo's notice of as-

<Page>

                                      -82-

            serted tax liability, notify Investor NewCo of its intent to do so,
            and Investor NewCo shall cooperate and shall cause the Tax
            Indemnification Companies or their respective successors to
            cooperate, at Hoechst's expense, in each phase of such Contest. If
            Hoechst does not elect to direct the Contest or fails to notify
            Investor NewCo of its election as herein provided, Investor NewCo or
            the Tax Indemnification Companies may pay, compromise or contest
            such asserted tax liability; PROVIDED that neither Investor NewCo
            nor the Tax Indemnification Companies may settle or compromise any
            asserted tax liability over the objection of Hoechst (which consent
            to settlement or compromise shall not be unreasonably withheld). In
            any event, Hoechst may participate, at its own expense, in any
            Contest. If Hoechst chooses to direct the Contest, Investor NewCo
            shall promptly empower and shall cause the Tax Indemnification
            Companies or their respective successor promptly to empower (by
            power of attorney and such other documentation as may be necessary
            and appropriate) the designated representatives of Hoechst to
            represent Investor NewCo or the Tax Indemnification Companies or
            their successor in the Contest insofar as the Contest involves an
            asserted tax liability for which Hoechst would be liable under
            Section 8.1.

      8.7   CONVEYANCE TAXES. Subject to Section 8.1(a)(B), Investor NewCo
            agrees to assume liability for and to pay all sales, transfer,
            stamp, stock transfer, real property transfer or similar taxes
            (excluding, for the avoidance of doubt, any income or similar taxes
            such as without limitation any corporate income tax, trade tax and
            solidarity surcharge, collectively referred to as "INCOME TAX")
            incurred as a result of the transactions contemplated hereby;
            PROVIDED that any such Income Taxes shall be borne by Hoechst, if
            and to the extent they relate to the consummation of the Hoechst
            NewCo II Contribution Agreement, the Hoechst NewCo II Share Purchase
            Agreement I, the Share Loan Agreement, the Call Option, the
            Counter-Call Option or Counter-Put Option and the Forward Sale
            Agreements I and II and that Hoechst shall indemnify without any
            limitations Investor NewCo, Hoechst NewCo II and Hoechst NewCo III
            (after exercise of the Counter-Call Option or the Counter-Put
            Option) from any such Income Taxes.

<Page>

                                      -83-


            It is agreed and Investor NewCo shall procure that during the
            Call-Option Period, the Hoechst NewCo II Shares, which are subject
            to the Call Option, will not constitute more than 70% of the
            outstanding share capital of Hoechst NewCo II for real estate
            transfer tax purposes.

      8.8   MISCELLANEOUS.

            (a)   To the extent permitted by law, the parties agree to treat all
                  payments made under this Section 8, under any other indemnity
                  provision contained in this Agreement, and for any
                  misrepresentations or breach of warranties or covenants, as
                  adjustments to the Main MGG Purchase Price for all tax
                  purposes.

            (b)   For purposes of this Section 8, all references to Investor
                  NewCo, Hoechst and the Tax Indemnification Companies include
                  successors.

            (c)   The obligations of the parties pursuant to this Section 8
                  shall survive the Closing Date and shall be time-barred six
                  (6) months after the tax assessment for the respective Tax
                  Indemnification Company for the respective tax period has
                  become final; PROVIDED that claims of Hoechst only become
                  time-barred if Investor NewCo has duly informed Hoechst of any
                  claims Hoechst may have under this Section 8. The limitation
                  period stops running if claims are asserted by one party
                  against the other on the basis of this Section 8 and if the
                  reasons on which such claim is based are reasonably defined.

      8.9   CLARIFICATIONS. It is understood that any Tax payments resulting
            from the consummation of the Foreign Sub Contribution Agreement are
            not subject to any indemnification claims under this Agreement.
            Accordingly, each party shall bear its own Tax liabilities arising
            from the Foreign Sub Contribution Agreement.

      8.10  AVENTIS CHINA COMPANIES. In case of indemnifiable Taxes incurred by
            either ACIC, New China Holding (as defined in the China SPA), any of
            the ACIC Gas Companies (as defined in the China SPA and being

<Page>

                                      -84-


            identical with the Aventis China Companies), any other Chinese
            company sold to MGG directly or indirectly in connection with any
            sale and transfer mechanism or the MCCO Assets, Sections 8.1 through
            8.9 shall apply MUTATIS MUTANDIS; provided that the 66%-Principle
            shall not apply but the Pro Rata Shareholding Principle shall apply
            and that any indemnifications shall be made by Hoechst to MGG.

9.    ENVIRONMENTAL MATTERS

      9.1   REFUND AND ALLOCATION OF DAMAGE CONTROL EXPENSE. Any Damage Control
            Expenses incurred after the Financial Date shall be allocated and
            refunded by Hoechst according only to the following rules with the
            limitations and restrictions set forth in Section 5 and 6 not being
            applicable, except for Section 6.3(c) last sentence.

            (a)   The allocation of Damage Control Expenses shall be based upon
                  the principle that (i) Hoechst shall only bear a certain quota
                  of any and all Damage Control Expenses (the "HOECHST DAMAGE
                  CONTROL EXPENSE QUOTA"), which quota shall be based upon the
                  rules explained under (b) below, that (ii) except for
                  specified cases, a de minimis amount and a threshold must be
                  observed as explained under (c) below, and that (iii)
                  additionally a cap shall apply as explained under (d) below.

            (b)   The Hoechst Damage Control Expense Quota shall be computed as
                  follows:

                  (i)   Subject to (iii) below, the maximum amount to be
                        allocated to Hoechst as the Hoechst Damage Control
                        Expense Quota shall be sixty-six and two/thirds percent
                        (66 2/3%)) of all Damage Control Expenses incurred by
                        any Fully Owned Environmental Company.

                  (ii)  Subject to (iii) below, if a Non-Fully Owned
                        Environmental Company incurs Damage Control Expenses,
                        the maximum amount to be allocated to Hoechst as the
                        Hoechst Damage Control Expense Quota is subject to the
                        Pro Rata Principle. For example: In case of a 51% owned

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                                      -85-


                        Environmental Company, the Hoechst Damage Control
                        Expense Quota amounts to thirty-four percent (34%)
                        (= 66 2/3 x 51%).

                  (iii) Of the amounts computed pursuant to (i) and (ii) of this
                        paragraph and as the Hoechst Damage Control Expense
                        Quota, Hoechst shall bear eighty percent (80%); PROVIDED
                        that Hoechst shall bear 100% of the amounts so computed
                        in case of Damage Control Expenses for Environmental
                        Contaminations on real estate which any time prior to
                        Closing had been owned but which is neither owned nor
                        occupied by an Environmental Company at the Closing Date
                        (the "FORMER MESSER REAL ESTATE"); and further PROVIDED
                        that Hoechst shall bear 90% of the amounts computed
                        pursuant to (i) and (ii) in case of Environmental
                        Contaminations attributable to the five sites
                        identified in table 1 of EXHIBIT 9.1 as long as and to
                        the extent that the aggregate Damage Control Expenses
                        resulting from Environmental Contaminations attributable
                        to those five sites do not exceed the amount of
                        EUR 26,000,000 and regarding amounts in excess thereof,
                        Hoechst shall bear 80%.

            (c)   Investor NewCo may in any event only claim a refund of Damage
                  Control Expenses if the aggregate amount of the Hoechst Damage
                  Control Expense Quota attributable to a Site exceeds the
                  amount of EUR 50,000 (the "ENVIRONMENTAL DE MINIMIS AMOUNT")
                  and if, furthermore, the aggregate amount of the Hoechst
                  Damage Control Expense Quotas above the Environmental De
                  Minimis Amount exceeds the amount of EUR 4,000,000 (the
                  "ENVIRONMENTAL THRESHOLD"); PROVIDED that the Environmental De
                  Minimis Amount and the Environmental Threshold shall not apply
                  to Damage Control Expenses attributable to (i) Former Messer
                  Real Estate, (ii) the five sites identified in table 1 of
                  EXHIBIT 9.1 and (iii) the sites identified in table 2 of
                  EXHIBIT 9.1.

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                                      -86-


            (d)   In no event shall Hoechst's aggregate obligation under this
                  Section 9.1 exceed an amount of fifteen percent (15%) of the
                  Total Compensation (the "ENVIRONMENTAL CAP"); PROVIDED that
                  the Environmental Cap shall not apply to Damage Control
                  Expenses attributable to (i) Former Messer Real Estate and
                  (ii) the five sites identified in table 1 of EXHIBIT 9.1.

            (e)   Hoechst shall not have an obligation under this Section 9.1 if
                  and up to the amount the underlying circumstances

                  (i)   are provided for in the Financial Statements 1999,

                  (ii)  are provided for in the individual audited financial
                        statement of the Environmental Company affected as of
                        December 31, 1999, or

                  (iii) are the subject of valid and collectable claims for
                        corresponding performances of third parties, in
                        particular by insurances (the validity risk and the
                        insolvency risk and all Losses reasonably associated
                        with the collection of such claims regarding the third
                        party shall be borne by Hoechst).

      9.2   CONDITIONS TO CERTAIN REFUNDS. The refund of Damage Control Expenses
            under Section 9.1 is conditioned on the following:

            (a)   Subject to the Environmental Companies' legal obligations,
                  Investor NewCo shall reasonably endeavor, without incurring
                  specific expenses not being (partially) reimbursed hereunder,
                  to limit the scope of any Damage Control Measures, including,
                  without limitation, initiating and diligently pursuing all
                  reasonable actions challenging threatened or issued orders of
                  Governmental Entities with respect to the determination of any
                  Environmental Contamination or of any Environmental Damage and
                  to Damage Control Measures (the "GOVERNMENTAL ORDERS") and
                  diligently pursuing (after prompt notice to and consent by
                  Hoechst, which shall not be unreasonably withheld or delayed,
                  it being understood that Investor NewCo shall bear the cost of
                  any such third party, private litigation undertaken without
                  the consent of

<Page>

                                      -87-

                  Hoechst, unless Hoechst was obliged to consent) any reasonable
                  third party private litigation. Investor NewCo shall not
                  settle any matter relating to an Environmental Contamination
                  without the written consent of Hoechst which consent shall not
                  be unreasonably withheld. Investor NewCo shall ensure that any
                  improvements are designed and constructed in such a manner as
                  to reasonably mitigate Damage Control Expenses. Without
                  prejudice to the foregoing, the parties are in Agreement that
                  a good faith cooperation with the public authorities is the
                  preferred manner to mitigate Damage Control Expenses.

            (b)   Unless the Damage Control Measure is required by specific
                  Governmental Order, prior to undertaking any Damage Control
                  Measures, Investor NewCo shall obtain a report from an
                  environmental professional expert in the subject matter of
                  Environmental Contamination (an "EXPERT") regarding the kind
                  and extent of the Environmental Contamination and of the
                  Damage Control Measure, the results of which report shall
                  create a rebuttable presumption regarding the extent of the
                  Environmental Contaminations. If the circumstances warrant
                  that a Damage Control Measure must be undertaken immediately
                  under Environmental Law to avoid imminent danger to human
                  health or consequences under criminal law on an emergency
                  basis (GEFAHR IN VERZUG) and it is not possible to obtain an
                  Expert opinion prior to undertaking such Damage Control
                  Measure, the requirement set forth in the immediately
                  preceding sentence shall be complied with as soon as possible.
                  Any Expert shall be selected jointly by Investor NewCo and
                  Hoechst. In the event that Hoechst and Investor NewCo cannot
                  agree on an Expert, the Expert shall be selected by the
                  President of the Chamber of Commerce in Frankfurt am Main. The
                  report of the Expert, once approved by Hoechst and Investor
                  NewCo, may be disclosed by Investor NewCo or Hoechst to third
                  parties, including Governmental Entities. Notwithstanding any
                  other provision of this Agreement, the costs of retaining the
                  Expert shall be borne equally by Investor NewCo and Hoechst,
                  unless the involvement of the Expert is required

<Page>

                                      -88-


                  anyway, in which case the costs of the Expert shall be
                  regarded as Damage Control Expenses.

            (c)   Prior to incurring any Damage Control Expense required
                  pursuant to a remedial action plan (a "RAP"), Investor NewCo
                  shall provide Hoechst with a draft of a proposed RAP at least
                  twenty (20) Business Days before disclosure to any
                  Governmental Entity or third party claimants and shall
                  consider and incorporate when appropriate and consistent
                  with this Agreement any comments and suggested modifications
                  provided in writing within such 20-day period. Investor NewCo
                  shall not implement any RAP with respect to a Damage Control
                  Measure without the prior consent of Hoechst, which consent
                  shall not be unreasonably withheld or delayed; PROVIDED,
                  HOWEVER, that in the case of a Damage Control Measure
                  undertaken in an emergency, a RAP may promptly be provided to
                  the competent Governmental Entity. A RAP shall contain at
                  least a description of the actions that will be undertaken in
                  the Damage Control Measure and an estimation of the expenses
                  that will be incurred in respect of the Damage Control
                  Measure.

            (d)   Contractors hired to complete a Damage Control Measure in
                  excess of EUR 500,000 shall be chosen according to customary
                  bidding procedures from at least three (3) qualified
                  independent companies selected by Investor NewCo to be invited
                  to bid and reasonably acceptable to Hoechst. If, in the
                  opinion of Hoechst and Investor NewCo, the bidders are of
                  equal quality, expertise and experience, and the proposals of
                  the bidders are sufficiently precise to permit direct
                  comparison, the lowest bidder shall be awarded the contract.

            (e)   If Investor NewCo or any of the Messer Group Companies become
                  involved in a governmental or third party claim or actions
                  that is likely to result in an obligation of Hoechst to
                  indemnify Investor NewCo under this Section 9, Investor NewCo
                  shall notify Hoechst of such claim or action. In the event of
                  a claim or action that is reasonably likely to result in a
                  claim for Damage Control Expenses in excess of EUR 500,000,
                  Investor NewCo

<Page>

                                      -89-


                  shall procure that Hoechst and/or its advisors shall have a
                  reasonable opportunity to participate in the process of
                  evaluating the remedial actions to be taken in connection with
                  the underlying Environmental Damage, including by
                  participating in material communications with Governmental
                  Entities or third party claimants (or by receiving prompt
                  notice in reasonable details about any such material
                  communications that were not prearranged).

            (f)   Hoechst shall always be notified in advance of the selection
                  of any Damage Control Measure exceeding EUR 100,000 and for
                  any such Damage Control Measure likely to exceed EUR 500,000
                  shall be afforded the opportunity to participate in decision
                  making regarding the structure and implementation of any
                  Damage Control Measure, including the bidding process related
                  thereto upon reasonable prior notice.

            (g)   There is no refund obligation of Hoechst under this Section 9
                  with respect to any Damage Control Expenses if and to the
                  extent such Damage Control Expenses would not have arisen but
                  for the change of a use of a Site after the Closing Date.

      9.3   POST-CLOSING ENVIRONMENTAL CONTAMINATION.

            (a)   In the event that a Damage Control Measure remedies
                  environmental contamination that is comprised of (i)
                  Environmental Damage and (ii) environmental contamination
                  created after the Closing Date, the obligation of Hoechst
                  pursuant to Section 9.1(b) hereof shall be only in respect
                  of the Environmental Damage; PROVIDED that any environmental
                  contamination discovered within one year after the Closing
                  Date that is not the result of a known release or spill after
                  the Closing Date (to be confirmed in writing by Investor
                  NewCo) shall be presumed to be Environmental Contamination
                  (subject to the demonstration by Hoechst to the contrary). In
                  the event that Investor NewCo and Hoechst cannot agree as to
                  the amount of the Environmental Contamination, the Company
                  will with the consent of Hoechst

<Page>
                                      -90-


                  and Investor NewCo retain an Expert to prepare a report
                  delineating the applicable proportion, the results of which
                  report shall create a rebuttable presumption.

            (b)   Investor NewCo shall immediately notify Hoechst of (i) any
                  environmental contamination (particularly environmental
                  contamination resulting from spills and releases
                  occurring/caused after the Closing Date) and (ii) of any
                  Environmental Contamination discovered after the Closing Date.

      9.4   APPLICATION FOR REFUND. Investor NewCo shall apply to Hoechst for
            the refund of Damage Control Expenses set forth in Section 9.2
            hereof as follows:

            (a)   Investor NewCo shall request from Hoechst reimbursement of
                  identified amounts in a letter (a "REIMBURSEMENT
                  APPLICATION"), which Reimbursement Application shall include
                  if applicable: (i) a copy of all invoices, receipts and other
                  documentation (including claims) upon which the demand is
                  based; (ii) a copy of the applicable contractor agreement and
                  payment terms, and (iii) a receipt indicating the amounts
                  disbursed to the applicable contractors together with
                  contractor wiring instructions (if applicable).

            (b)   The refund of the Damage Control Expenses will be paid by
                  Hoechst within one (1) month after receiving the
                  Reimbursement Application unless a Reimbursement Application
                  is not sufficiently substantiated.

            (c)   Subject to the pendency of any action undertaken pursuant to
                  Section 9.2 hereof, Hoechst's obligation in respect of
                  Section 9.1(a) hereof shall be time-barred as follows:
                  Investor NewCo has notified Hoechst in writing within four (4)
                  years and six (6) months, and in case Damage Control Expenses
                  attributable to Former Messer Real Estate within ten (10)
                  years, after the Closing Date that it or any Environmental
                  Company (i) has received a court order or other Governmental
                  Order or other notification from a Governmental Entity or (ii)
                  a third party claim, or

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                                      -91-


                  (iii) has commenced with Damage Control Measures as a result
                  of a respective legal obligation, in each case reasonably
                  expected to give rise to Damage Control Expenses indemnifiable
                  under this Section 9. Hoechst shall not be responsible for
                  Damage Control Expenses to the extent that a Reimbursement
                  Application has not been received within twelve (12) months
                  after the date of any invoice for which Investor NewCo seeks
                  reimbursement.

      9.5   AVENTIS CHINA COMPANIES. In case of refundable Damage Control
            Expenses incurred by either ACIC, New China Holding (as defined in
            the China SPA), any of the ACIC Gas Companies (as defined in the
            China SPA and being identical with the Aventis China Companies), any
            other Chinese company sold to MGG directly or indirectly in
            connection with any sale and transfer mechanism or the MCCO Assets,
            Sections 9.1 through 9.4 shall apply MUTATIS MUTANDIS; PROVIDED that
            the 66%-Principle shall not apply but the Pro Rata Shareholding
            Principle shall apply and that any indemnifications shall be made by
            Hoechst to MGG.

10.   TERMINATION

      10.1  TERMINATION. This Agreement may be terminated at any time prior to
            the Closing:

            (a)   by Hoechst or Investor NewCo if the Closing shall not have
                  occurred on or before March 31, 2001 (the "CLOSING DROP
                  DATE"); PROVIDED that the right to terminate this Agreement
                  under this Section 10.1(a) shall not be available to any
                  party whose failure to fulfill any obligation under the
                  Combination Documents shall have been the cause of, or shall
                  have resulted in, the failure of the Closing to occur prior
                  to such date;

            (b)   by the mutual written consent of Hoechst and Investor NewCo;
                  or

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                                      -92-


            (c)   by Investor NewCo if it has been determined in accordance with
                  Section 3.4 that the closing condition set forth in Section
                  3.1(f) cannot be satisfied.

      10.2  SPECIAL TERMINATION RIGHT OF INVESTOR NEWCO. Notwithstanding any
            provision of this Agreement to the contrary, it is acknowledged and
            agreed that Investor NewCo has not, as of the date of the execution
            of this Agreement, obtained binding commitments for the financing
            (including with respect to the Loan Notes) necessary to effect the
            transactions contemplated by this Agreement (the "FINANCING"), which
            financing will necessarily include all funds necessary for payment
            of the Total Compensation, the refinancing of existing indebtedness
            of the Messer Group Companies, the provision of financing
            commitments with respect to contingent liabilities of the Messer
            Group Companies, and the payment of all fees and expenses relating
            to the transaction. Investor NewCo agrees to use reasonable
            endeavors to obtain, as promptly as practicable following the
            execution of this Agreement, one or more legally binding and
            definitive commitments with respect to, in the aggregate, all of the
            Financing upon terms and conditions, and in amounts, satisfactory to
            Investor NewCo and its shareholders, which terms and conditions will
            necessarily include that such commitments are subject to no material
            initial funding conditions other than the satisfaction of the
            conditions set forth in Section 3.1 (a commitment or commitments
            with respect to, in the aggregate, all of the Financing and
            satisfying the foregoing criteria being referred to as a "DEFINITIVE
            FINANCING COMMITMENT"). In the event that at any time on or before 8
            February 2001, Investor NewCo determines and notifies Hoechst that,
            for any reason, a Definitive Financing Commitment will not be or is
            not likely to be available, or that the Financing contemplated by a
            Definitive Financing Commitment will not be or is not likely to be
            available on the Closing Date, Investor NewCo shall have the right
            to terminate this Agreement, without liability or penalty. In no
            event shall Investor NewCo be deemed to have breached its
            obligations under this Agreement as a result of any inability by it
            to obtain a Definitive Financing Commitment, PROVIDED that Investor
            NewCo has used reasonable endeavors to obtain such a Definitive
            Financing Commitment. After 8 February 2001, Investor NewCo is no
            longer enti-

<Page>

                                      -93-


            tled to terminate this Agreement on the basis of this
            Section 10.2 unless such date has been extended by mutual written
            agreement of Investor NewCo and Hoechst.

            It is acknowledged that the terms of a Definitive Financing
            Commitment, and the ability of Investor NewCo to obtain a Definitive
            Financing Commitment, may be impacted by, among other things, the
            results of operations and financial condition of MGG for the 2000
            fiscal year as reflected in Financial Statements 2000. It is
            acknowledged that the ability of Investor NewCo to obtain a
            Definitive Financing Commitment prior to 8 February 2001 as
            contemplated by the foregoing assumes that Investor NewCo will
            receive a draft of the audited Financial Statements 2000 by no later
            than 29 January 2001; PROVIDED that Hoechst does not undertake any
            responsibility for the timely delivery of such accounts.

      10.3  SPECIAL TERMINATION RIGHT OF HOECHST

            (a)   In the event that at any time on or before 24 January 2001,
                  Hoechst determines and notifies Investor NewCo that it has,
                  for whatever reason, not received a satisfactory, in its sole
                  discretion, amendment to a first tax ruling already received,
                  Hoechst shall have the right to terminate this Agreement,
                  without liability or penalty. In no event shall Hoechst be
                  deemed to have breached its obligations under this Agreement
                  as a result of it not having received a satisfactory amendment
                  to the above-mentioned tax ruling; PROVIDED that Hoechst has
                  used reasonable endeavors to obtain such an amendment. After
                  24 January 2001, Hoechst is no longer entitled to terminate
                  this Agreement on the basis of this Section 10.3 unless such
                  date has been extended by mutual agreement of Investor NewCo
                  and Hoechst.

            (b)   In the event that based on the delivered Financial Statements
                  2000 it is revealed that Hoechst would have to pay for a
                  breach of Section 5.19 indemnification in excess of EUR
                  20,000,000, Hoechst may terminate this Agreement prior to the
                  Closing Date, without liability or penalty.

<Page>

                                      -94-


      10.4  EFFECT OF TERMINATION. In the event of any termination of this
            Agreement as provided in Sections 10.1 through 10.3, this Agreement
            shall forthwith become void and there shall be no liability on the
            part of either party hereto to any other party hereto except (i)
            that the provisions of this Section 10 and Sections 12.4 and 12.5
            through 12.15 except 12.14 shall survive termination of the
            Agreement, and (ii) that nothing herein shall relieve either party
            from liability for any breach of this Agreement.

11.   GUARANTEES

      Hoechst hereby assumes full responsibility by way of an independent
      guarantee (SELBSTANDIGE ERFULLUNGSGARANTIE) that ACIC and, if applicable
      and during the periods they are controlled by Hoechst, Hoechst NewCo II
      and Hoechst NewCo III fully and timely comply with all obligations assumed
      under the Combination Documents. Hoechst will procure that Aventis will
      deliver at Closing a guarantee substantially in the form of EXHIBIT
      3.1(j).

12.   ADDITIONAL AGREEMENTS

      12.1  MIG RIGHTS. MIG hereby agrees that the transactions contemplated by
            this Agreement will be consummated, and waives all rights of first
            refusals or other preference rights it may have with respect to the
            sale and transfer of the Hoechst MGG Share. MIG shall and Hoechst
            shall procure that all legal formalities required under the articles
            of association of MGG will be fulfilled to implement the
            transactions contemplated by this Agreement. Subject to the
            foregoing, nothing contained in this Agreement shall be construed
            such as that MIG waives any of its rights under the articles of
            association of MGG. Hoechst and MIG are in agreement that with
            effect as of the Closing Date, the GRUNDSATZVEREINBARUNG, as
            amended, shall terminate. Any rights of MIG and Hoechst under said
            GRUNDSATZVEREINBARUNG remains otherwise in full force and effect
            until the Closing Date. For the avoidance of doubt: If for any
            reason the Closing Date does not occur, rights and obligations of
            Hoechst and MIG under the articles of association and said
            GRUNDSATZVEREIN-

<Page>

                                      -95-


            BARUNG remain unaffected. In the event that the Call Option is
            exercised but neither the Counter-Call Option nor the Counter-Put
            Option is exercised, Hoechst and MIG shall treat each other as if
            the current articles of incorporation of MGG and the
            GRUNDSATZVEREINBARUNG came again in force.

      12.2  NON-COMPETE. Subject to any permitted holding of interests in MGG
            under this Agreement by Hoechst, Hoechst hereby undertakes for
            itself, Aventis and any of Aventis' Subsidiaries (other than the
            Messer Group Companies) for a period of three (3) years following
            the Closing Date (i) not to conduct any activity within the
            territorial scope of the Gas Business of the Messer Group Companies
            as of the date hereof which would directly or indirectly compete
            with the Gas Business as conducted by the Messer Group Companies as
            of the date hereof or which would directly or indirectly result in
            such competition, (ii) not to acquire any interest in any Person, a
            core business of which (i.e., more than 33% of such Person's annual
            sale) is, directly or indirectly, to compete with the Gas Business
            of the Messer Group Companies, and (iii) not to solicit any current
            employee of the Messer Group; PROVIDED, however, that (i) the
            current activities of any affiliate of Hoechst and of its parent
            Aventis with the exception of the Messer Group Companies shall not
            constitute a competing conduct of Hoechst, (ii) Hoechst and its
            parent Aventis may directly or indirectly hold interests or
            securities of any person solely as passive investment to the extent
            such investment cannot confer to Aventis directly or indirectly more
            than one-third of the voting rights of such Person, and (iii)
            Aventis and its Subsidiaries may acquire interests in companies
            competing with the Gas Business with annual sales in the gas
            business of no more than EUR 25,000,000 which competing gas business
            must then be sold within nine (9) months.

      12.3  EXPENSES AND FEES. Except as otherwise specifically provided in this
            Agreement including the Combination Documents, each party shall pay
            and bear its own expenses and fees (including attorneys',
            accountants', consultants' and advisors' fees) in connection with
            this Agreement or any of the transactions contemplated hereby. The
            cost of the notarization of this Agreement shall be borne by
            Hoechst. Any fees and costs of merger

<Page>

                                      -96-


            control procedures (excluding lawyer's fees) connected with the
            execution and consummation of this Agreement, shall be borne by
            Investor NewCo. The notarization costs of the Hoechst NewCo III
            Share Purchase Counter-Call Agreement or the Hoechst NewCo III Share
            Purchase Counter-Put Agreement shall be borne by Hoechst. In
            addition Hoechst shall pay to the parties(on a pro rata
            basis-headcount) of a certain KONSORTIALVEREINBARUNG an amount of
            50% of the notarial fees arising from the notarization of such
            KONSORTIALVEREINBARUNG which is being notarized of even date by and
            between, among others, Allianz Capital Partners GmbH, GS Funds and
            MIG, to the extent they exceed SFR 25,000 plus V.A.T.

      12.4  NOTICES. All notices hereunder shall be given to the respective
            parties hereto by hand delivery or by registered letter with receipt
            confirmed ("EINSCHREIBEN") and shall be considered delivered in all
            respects when delivered as follows:

            To Hoechst:

                  Hoechst Aktiengesellschaft
                  Industriepark Frankfurt am Main - Hochst
                  D-65926 Frankfurt am Main
                  Germany
                  Attention: Vorstand


            With a copy to:

                  Aventis
                  67917 Strasbourg, Cedex 9
                  France
                  Attention: General Counsel


            To Investor NewCo:

                  c/o Allianz Capital Partners GmbH
                  Theresienstr. 1-7,
                  80333 Munchen
                  Germany
                  Attention: Stefan Sanne

<Page>

                                      -97-


                  Goldmann Sachs International
                  Peterborough Court
                  133 Fleet Street
                  London, EC4A 2BB
                  Great Britain
                  Attention: Ulrika Werdelin


            To MGG:

                  Messer Griesheim GmbH
                  Corporate Office
                  Frankfurt Airport Center 1, C9
                  D-60547 Frankfurt
                  Germany
                  Attention: Geschaftsfuhrung


            To MIG:

                  Messer Industrie Gesellschaft mbH
                  Hardtberg 1
                  61462 Konigstein
                  Attention: Geschaftsfuhrung

            or to such other address as any party hereto shall have designated
            by written notice to the other party from time to time.

      12.5  AMENDMENTS. Any amendment of or supplement to this Agreement,
            including this provision and the Exhibits, must be in writing to be
            valid, and must be notarized if required by law.

      12.6  HEADINGS. In this Agreement the headings are inserted for
            convenience only and shall not affect the interpretation of this
            Agreement. Where a German term has been inserted in brackets it
            alone shall be authoritative for the purpose of the interpretation
            of the relevant English term in this Agreement.

      12.7  DEFAULT INTEREST RATE. In the event that one of the parties to this
            Agreement is in default (VERZUG) with payments under this Agreement,
            it shall pay default interest at a rate of EURIBOR (three months)
            prevailing at the respective due date and as adjusted from time to
            time

<Page>

                                      -98-


            to time thereafter plus 4% p.a. (in addition to Contractual
            Interest, if due). Sec. 284 para 3 of the German Civil Code (BGB)
            shall not apply. Interest shall be compounded quarterly and shall be
            calculated on the basis of actual days elapsed divided by 360. The
            right to claim further damages, if any, shall remain unaffected.

      12.8  RIGHTS OF RETENTION. Any rights of retention are excluded. Unless
            undisputed or confirmed by a non-appealable decision, claims may not
            be set off against the Main MGG Purchase Price or any other payment
            claim pursuant to this Agreement.

      12.9  COMPLETE AGREEMENT. This Agreement constitutes the full
            understanding of the parties and the complete and exclusive
            statement of the terms and conditions of the Agreement relating to
            the subject matter hereof and supersedes any and all prior
            agreements, whether written or oral, that may exist between the
            parties with respect thereto.

      12.10 SUCCESSORS. This Agreement shall inure to the benefit of and be
            binding upon the parties and the respective successors and assigns
            of the parties hereto only (i.e., no third party beneficiary). This
            Agreement or any rights and obligations hereunder shall not be
            assigned to any other person by any party hereto without the prior
            written consent of the other parties hereto, other than as expressly
            agreed herein; PROVIDED that Investor NewCo may assign rights under
            this Agreement to financing institutions for security purposes or to
            Allianz AG or the Goldman Sachs Group, Inc. or any of their
            Subsidiaries or funds managed by them, which must remain
            Subsidiaries or funds managed by Affiliates of the Allianz AG or
            Goldman Sachs Group, Inc., and which must not further assign any
            such rights other than to a purchaser of any shares in Investor
            NewCo, who cannot exercise any such assigned rights differently than
            any of the aforesaid Persons. Nothing herein contained shall prevent
            an assignment hereof to a successor (GESAMTRECHTS-ODER
            EINZELRECHTS-NACHFOLGE) of any party if such succession is created
            as a result of a merger or consolidation involving a transfer of
            ownership of all or substantially all of its assets by any party
            hereto; PROVIDED that the successor to such party in any such
            transaction shall assume in writing or as a matter of law the
            obligations of such party hereunder with full continu-

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                                      -99-


            ing liability of such party and further provided that prior written
            notice of such transaction shall be given by such party to the other
            parties hereto. No assignment shall relieve any party hereto of its
            obligations hereunder.

      12.11 COUNTERPARTS. Each of the parties shall execute and deliver all such
            further documents and agreements and do such further acts as are
            reasonably required hereby and are not inconsistent with any other
            provisions of this Agreement.

      12.12 CONFIDENTIALITY

            (a)   The parties agree to keep confidential all information
                  obtained in connection with this Agreement (in the past or
                  future) and its performance as long as and to the extent that
                  it has not been made known to the public without fault of any
                  of the parties; PROVIDED, however, that no party shall be
                  restricted from sharing information with governmental or other
                  regulatory authorities to the extent legally required and that
                  Investor NewCo, Hoechst NewCo II and MGG shall not be
                  restricted from sharing information with financial
                  institutions involved in the financing of the transactions
                  contemplated herein including the refinancing of presently
                  existing debt, syndication of debt and preparation, filing and
                  publication of prospectus for the issuance of a high
                  yield bond or other financing instruments. If this Agreement
                  is terminated or rescinded for any reason, this
                  confidentiality obligation shall remain in effect
                  indefinitely. After signature of this Agreement, the parties
                  shall jointly issue a press release. Neither party shall,
                  without the prior consultation of the other, issue any oral or
                  written statement to the press or to the public regarding this
                  Agreement, except as required by law or the rules of any stock
                  exchange or governmental or other regulatory authority,
                  including the requirement to make a disclosure of the value of
                  the consideration in the next published accounts of either
                  party. Nothing in this Section 12.12(a) shall be understood to
                  prevent any party from filing a claim against the other party
                  before court on the basis of this Agreement.

<Page>

                                     -100-


            (b)   In addition to Section 12.12(a) and except as otherwise
                  provided in this Agreement, after the date of this Agreement
                  Hoechst and its Affiliates shall not use or disclose to third
                  parties any information disclosed, transferred, assigned,
                  licensed or otherwise made available by or to Investor NewCo
                  hereunder and relating to the Gas Business, unless (i) such
                  information is needed by Hoechst to continue its commercial
                  activities or to execute its rights or comply with its
                  obligations hereunder, (ii) is or becomes public knowledge
                  through no fault of Hoechst, (iii) is passed to Hoechst after
                  the Closing Date by a third party which is under no obligation
                  of confidentiality, or (iv) has to be disclosed by Hoechst
                  pursuant to law, judicial or official order; in such case
                  Hoechst shall notify Investor NewCo not less than 10 (ten)
                  days before making the impending disclosure.

      12.13 SEVERABILITY CLAUSE. If any provision of this Agreement should be
            invalid or unenforceable in whole or in part for any reason
            whatsoever, the remaining provisions of this Agreement shall not be
            affected thereby. The invalid or unenforceable provision shall be
            replaced with a valid and enforceable provision renegotiated in good
            faith in order to achieve the intent of the parties to the fullest
            extent possible. If this Agreement should contain a contractual gap,
            such reasonable provision shall apply for the purpose of filling
            such contractual gap, which the parties had intended in view of the
            purpose of this Agreement if they had considered the issue.

      12.14 ACCESS TO INFORMATION. Investor NewCo shall procure that Hoechst or
            its advisors shall have reasonable access to the employees,
            documents, books and records of each of the Messer Group Companies
            including any successors, so that Hoechst can fully enforce any
            rights and determine any obligations it may have under Sections 6,
            8, 9 and 10 of this Agreement or in relation to the Messer Group
            towards third parties or authorities. If and to the extent Hoechst
            may have any rights to receive additional payments of Investor
            NewCo, Investor NewCo shall procure that Hoechst will receive the
            necessary information without any further request.

<Page>

                                     -101-


      12.15 GOVERNING LAW. The Agreement shall be governed by the laws of
            Germany.

      12.16 ARBITRATION. All disputes arising in connection with this Agreement
            or its validity shall be finally settled according to the
            Arbitration Rules of the German Institution of Arbitration e.V.
            (DIS) without recourse to the ordinary courts of law except for
            interim proceedings (EINSTWEILIGER RECHTSCHUTZ). The place of
            arbitration shall be Frankfurt am Main. The arbitral tribunal shall
            consist of three arbitrators, appointed in accordance with said
            Arbitration Rules. The substantive law of Germany shall be
            applicable to the dispute. The language of the arbitral proceedings
            shall be English.

                                      *****

<Page>

                                      -102-


This deed and EXHIBIT 4.17 was read to the persons appearing in the presence of
the notary, approved by them and signed by them as well as the notary as
follows: