Exemptions from taxes for Federal travelers, and the forms required to claim them, vary from location to location. The GSA Per Diem Rates webpage (http://gsa.gov/perdiem) provides more information on State tax exemptions.
Code of Federal Regulations
[FTR Amdt. 70, 63 FR 15961, Apr. 1, 1998, as amended by FTR Amdt. 2007-05, 72 FR 61537, Oct. 31, 2007]