§ 31.3306(b)-2. Reimbursement and other expense allowance amounts.

¶(i) General rule. Except as provided in paragraph (b)(1)(ii) of this section, if a reimbursement or other expense allowance arrangement satisfied the requirements of section 62(c) and § 1.62-2, but the expenses are not substantiated within a reasonable period of time or amounts in excess of the substantiated expenses are not returned within a reasonable period of time, the amount paid under the arrangement in excess of the substantiated expenses is treated as paid under a nonaccountable plan, is included in wages, and is subject to withholding and payment of employment taxes no later than the first payroll period following the end of the reasonable period.

¶(ii) Per diem or mileage allowances. If a reimbursement or other expense allowance arrangement providing a per diem or mileage allowance satisfies the requirements of section 62(c) and § 1.62-2, but the allowance is paid at a rate for each day or mile of travel that exceeds the amount of the employee's expenses deemed substantiated for a day or mile of travel, the excess portion is treated as paid under a nonaccountable plan and is included in wages. In the case of a per diem or mileage allowance paid as a reimbursement, the excess portion is subject to withholding and payment of employment taxes when paid. In the case of a per diem or mileage allowance paid as an advance, the excess portion is subject to withholding and payment of employment taxes no later than the first payroll period following the payroll period in which the expenses with respect to which the advance was paid (i.e., the days or miles of travel) are substantiated. The Commissioner may, in his discretion, prescribe special rules in pronouncements of general applicability regarding the timing of withholding and payment of employment taxes on per diem and mileage allowances.