SIXTH AMENDMENT TO CREDIT AGREEMENT

Exhibit 10.1

EXECUTION VERSION

SIXTH AMENDMENT

TO

CREDIT AGREEMENT

THIS SIXTH AMENDMENT TO CREDIT AGREEMENT (this “ Amendment ”), is made and entered into on May 1, 2009 (“ Signature Date ”) and made effective as of March 31, 2009 (the “ Effective Date ”), by and among, on the one hand, CECO ENVIRONMENTAL CORP ., a Delaware corporation (“ Parent ”), CECO GROUP, INC ., a Delaware corporation (“ Group ”), FKI, LLC , a Delaware limited liability company (“ FKI, LLC ”), CECO MEXICO HOLDINGS LLC , a Delaware limited liability company (“ CECO Mexico LLC ”), AVC, INC., a Delaware corporation (“ AVC, Inc. ”), and each of the following Subsidiaries of Parent as Borrowers under this Amendment and the Credit Agreement: CECO FILTERS, INC ., a Delaware corporation (“ Filters ”), NEW BUSCH CO., INC ., a Delaware corporation (“ New Busch ”), THE KIRK & BLUM MANUFACTURING COMPANY , an Ohio corporation (“ K&B ”), KBD/TECHNIC, INC ., an Indiana corporation (“ Technic ”), CECOAIRE, INC ., a Delaware corporation (“ Aire ”), CECO ABATEMENT SYSTEMS, INC ., a Delaware corporation (“ Abatement ”), H.M. WHITE, INC ., a Delaware corporation (“ H.M. White ”), EFFOX INC ., a Delaware corporation and formerly known as CECO ACQUISITION CORP. (“ Effox ”), GMD ENVIRONMENTAL TECHNOLOGIES, INC. , a Delaware corporation and formerly known as GMD ACQUISITION CORP. (“ GMD ”), and FISHER-KLOSTERMAN, INC ., a Delaware corporation and formerly known as FKI ACQUISITION CORP. (“ Fisher-Klosterman ”), and, on the other hand, FIFTH THIRD BANK , an Ohio banking corporation (“ Lender ”), is as follows:

Preliminary Statements

A. Parent, Group, and Borrowers executed and delivered to Lender that certain Credit Agreement dated as of December 29, 2005, as amended by the First Amendment to Credit Agreement dated as of June 8, 2006, the Second Amendment to Credit Agreement dated as of February 28, 2007, the Third Amendment to Credit Agreement dated as of February 29, 2008, the Fourth Amendment to Credit Agreement dated as of August 1, 2008, and the Fifth Amendment to Credit Agreement (the “ Fifth Amendment ”) dated as of December 30, 2008 (as amended, the “ Credit Agreement ”). FKI, LLC and CECO Mexico LLC are additional parties to the Third Amendment, Fourth Amendment, and Fifth Amendment. Capitalized terms which are used, but not defined, in this Amendment will have the meanings given to them in the Credit Agreement.

B. Parent, Group, FKI, LLC, CECO Mexico LLC, AVC, Inc., and Borrowers (collectively, the “ Loan Parties ”) have requested that Lender: (i) extend the stated Termination Date of the Line of Credit from January 31, 2010 to April 1, 2011; (ii) make certain changes to the interest rates applicable to the Obligations, including, without limitation, the implementation of a daily reset, one-month LIBOR-based rate and the unavailability of a Prime-based rate except in certain circumstances; (iii) consent to a one-time payment of principal on the Subordinated Debt in the aggregate amount not to exceed $3,000,000, to be made by Parent to Subordinated Creditor on March 31, 2009 (the “ Subordinated Debt Payment ”) and consent to a one-time payment of an extension fee

with respect to the Subordinated Debt in the aggregate amount not to exceed CAD $38,220, to be made by Parent to Subordinated Creditor on the Signature Date (the “ Subordinated Debt Extension Fee ”); and (iv) make certain other amendments to the Credit Agreement and certain of the other Loan Documents.

C. Lender is willing to so amend the Credit Agreement and other Loan Documents, all on the terms, and subject to the conditions, of this Amendment.

Statement of Agreement

In consideration of the mutual covenants and agreements set forth in this Amendment, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, Lender and the Loan Parties hereby agree as follows:

1. Amendments to Credit Agreement . Subject to the satisfaction of the conditions of this Amendment, the Credit Agreement is hereby amended as follows:

1.1 Section 1.1 of the Credit Agreement is hereby amended by the addition of the following new definitions, in their proper alphabetical order, to provide in their entirety as follows:

“ AVC, Inc. ” means AVC, Inc., a Delaware corporation.

“ Daily LIBOR Rate ” has the meaning given in the Revolving Note.

“ Daily LIBOR Rate Loan ” means that portion of the Loans which, as of any date, bears interest at an interest rate per annum equal to the Daily LIBOR Rate plus the applicable margin as set forth in the applicable Note.

“ Sixth Amendment ” means the Sixth Amendment to this Agreement dated as of May 1, 2009, to be effective as of March 31, 2009.

“ Tranche LIBOR Rate ” has the meaning given in the Revolving Note.

“ Tranche LIBOR Rate Loan ” means that portion of the Loans which, as of any date, bears interest at an interest rate per annum equal to the Tranche LIBOR Rate plus the applicable margin as set forth in the applicable Note.

1.2 The following definitions in Section 1.1 of the Credit Agreement are hereby amended in their entirety by substituting the following in their respective places:

“ Affiliate ” means, as to any Person (the “ Subject Person ”), any other Person which, directly or indirectly, is in control of, is controlled by, or is under common control with, the Subject Person. For purposes of this definition, “control” of a Person means the power, direct or indirect, (a) to vote 5% or more of the securities (or other Ownership Interests) having voting power for the election of directors (or managers in the case of a limited liability company) of the Person or (b) otherwise to direct or cause the direction of the management and policies of the Person, whether by contract or otherwise. Without limiting the generality of the foregoing, each of the following will be deemed an Affiliate of a Borrower for purposes of this Agreement, Parent, Group, FKI, LLC, CECO Mexico LLC, CECO India, Fisher Klosterman Shanghai, CECO Environmental Mexico, CECO Environmental Services, AVC, Inc., and each officer and director of a Loan Party.

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“ Applicable Unused Line Fee Percentage ” means, as of any date, the applicable percentage shown in the applicable column in the table below based on the then applicable Fixed Charge Coverage Ratio. As of the Signature Date (as defined in the Sixth Amendment), the Applicable Unused Line Fee Percentage is 0.50% ( i.e. , Pricing Grid Level 3).

“ Borrower ” means each of Filters, New Busch, K&B, Technic, Aire, Abatement, H.M. White, CECO Acquisition (now known as Effox Inc.), GMD, FKI Acquisition and the Domestic Subsidiaries of Parent or Group hereafter becoming a party to this Agreement pursuant to Section 5.9(b) , and “ Borrowers ” means, collectively, Filters, New Busch, K&B, Technic, Aire, Abatement, H.M. White, CECO Acquisition (now known as Effox Inc.), GMD, FKI Acquisition and such additional Domestic Subsidiaries. To the extent a term or provision of this Agreement or any of the other Loan Documents is applicable to a “Borrower”, it is applicable to each and every Borrower unless the context expressly indicates otherwise. For the avoidance of doubt, none of FKI, LLC, CECO Mexico LLC, or AVC, Inc. shall be a Borrower.

“ Business Day ” means (a) any day on which commercial banks in Cincinnati, Ohio are required by law to be open for business and (b) with respect to all notices and determinations in connection with, and payments of principal and interest on, Loans bearing interest with reference to the Tranche LIBOR Rate or the Daily LIBOR Rate, any day (other than a Saturday or Sunday) on which commercial banks are open for business in New York, New York. Periods of days referred to in this Agreement will be counted in calendar days unless Business Days are expressly prescribed.

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“ Fixed Charges ” means, for the applicable period, the total (without duplication), in Dollars, of (all as determined on a consolidated basis in accordance with GAAP): (a) the principal amount of the Parent and its Subsidiaries’ long-term Indebtedness, in each case paid during the applicable period, including those under Term Loan Note C (other than any Excess Cash Flow Payment with respect to Term Loan C) and the Subordinated Debt Note (as defined in the Subordination Agreement) (whether classified, as of any date, as long-term Indebtedness); plus (b) scheduled capital lease payments by the Parent and its Subsidiaries during the applicable period; and plus (c) the Parent and its Subsidiaries’ aggregate cash interest expense for the applicable period, including interest paid on the Obligations, all capital lease obligations, the Subordinated Debt, and any other Indebtedness for the applicable period; provided, however , that the following amounts will be excluded for purposes only of determining Fixed Charges: (i) that portion of the Subordinated Debt which, with Lender’s prior consent, is converted into shares of the Parent as a result of the exercise of the conversion rights of the Subordinated Creditor under the Subordinated Debt Note and (ii) the principal payment on the Subordinated Debt made on or about March 31, 2009 in an amount not to exceed $3,000,000, made by Parent in accordance with Section 3.1.2(b) of the Subordination Agreement. The term “applicable period” in this definition means Test Period in the case of determining the Fixed Charge Coverage Ratio or the Maximum Total Funded Debt to Adjusted EBITDA Ratio and Fiscal Year in the case of determining Excess Cash Flow.

“ Guaranties ” means, collectively, the Borrower Guaranties, the Group Guaranty, the Parent Guaranty and each guaranty made by Fisher-Klosterman, FKI, LLC, GMD, CECO Mexico LLC and AVC, Inc. in favor of Lender and Lender’s Affiliates of the Obligations.

“ Loan Party ” and “ Loan Parties ” mean each of Borrowers, Group, Parent, FKI, LLC, CECO Mexico LLC, and AVC, Inc., and collectively, Borrowers, Group, Parent, FKI, LLC, CECO Mexico LLC, and AVC, Inc., respectively.

“ Subordinated Creditor ” means Icarus Investment Corp., formerly known as Can-Med Technology, Inc. and formerly doing business as Green Diamond Oil Corporation, an Ontario corporation, and, subject to the Subordination Agreement, its successors and assigns of the Subordinated Debt and any Person holding Refinancing Debt of the Subordinated Debt as permitted under this Agreement.

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“ Subordination Agreement ” means the Subordination Agreement between the Subordinated Creditor and Lender dated as of August 14, 2008, as amended by the First Amendment to Subordination Agreement dated as of March 31, 2009.

“ Termination Date ” means: (a) with respect to the Line of Credit, the Letter of Credit Obligations and the other Obligations (other than Term Loan C), the earlier of (i) April 1, 2011 and (ii) the date upon which the entire outstanding balance under the Revolving Note shall become due pursuant to the provisions hereof (whether as a result of acceleration by Lender or otherwise); and (b) with respect to Term Loan C, the earliest of (i) April 1, 2011, (ii) the date upon which the entire outstanding balance under Term Loan Note C shall become due pursuant to the provisions hereof (whether as a result of acceleration by Lender or otherwise), and (iii) the date upon which Term Loan C shall be repaid in full.

1.3 Clause (vii) of the definition of “ Eligible Accounts ” in Section 1.1 of the Credit Agreement is hereby amended in its entirety by substituting the following in its place:

(vii) Accounts owing from any single account debtor to the extent, as of any date, that the total amount of such account debtor’s Indebtedness to any one or more Borrowers exceeds 35% of the face amount (less maximum discounts, credits and allowances which may be taken by, or granted to, such account debtor in connection therewith) of the then outstanding Eligible Accounts of such Borrower or Borrowers;

1.4 Section 2.1(c) of the Credit Agreement is hereby amended in its entirety by substituting the following in its place:

(c) On the Signature Date (as defined in the Sixth Amendment) of the Sixth Amendment, Borrowers shall execute and deliver to Lender a Fourth Amended and Restated Revolving Credit Promissory Note in the form of Exhibit 2.1 attached to the Sixth Amendment (as amended, the “ Revolving Note ”), dated as of the Effective Date (as defined in the Sixth Amendment), in the principal amount of the Revolving Commitment, and bearing interest at such rates, and payable upon such terms, as specified in the Revolving Note. On the Signature Date, all Prime Rate Loans outstanding under the Revolving Note will be automatically converted to bear interest as Daily LIBOR Rate Loans.

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1.5 The sixth sen