CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

     We have issued our report dated April 22, 2008, accompanying the financial
statements of IPOX-30 Index Portfolio 2007-1 (included in Van Kampen Unit
Trusts, Series 634) as of December 31, 2007, and for the period from January 3,
2007 (Initial Date of Deposit) through December 31, 2007 then ended and the
financial highlights for the period from January 3, 2007 (Initial Date of
Deposit) through December 31, 2007, contained in this Post-Effective Amendment
No. 1 to Form S-6 (File No. 333-139095) and Prospectus.

     We hereby consent to the use of the aforementioned report in this
Post-Effective Amendment and Prospectus and to the use of our name as it appears
under the caption "Other Matters-Independent Registered Public Accounting Firm".

                               GRANT THORNTON LLP

New York, New York
April 22, 2008