Section 31.3121(a)-3 - Reimbursement and other expense allowance amounts(a)When excluded from wages. If a reimbursement or other expense allowance arrangement meets the requirements of section 62(c) of the Code and § 1.62-2 and the expenses are substantiated within a reasonable period of time, payments made under the arrangement that do not exceed the substantiated expenses are treated as paid under an accountable plan and are not wages. In addition, if both wages and the reimbursement or other expense allowance are combined in a single payment, the reimbursement or other expense allowance must be identified either by making a separate payment or by specifically identifying the amount of the reimbursement or other expense allowance.(b)When included in wages -(1)Accountable plans -(i)General rule. Except as provided in paragraph (b)(1)(ii) of this section, if a reimbursement or other expense allowance arrangement satisfies 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.(2)Nonaccountable plans. If a reimbursement or other expense allowance arrangement does not satisfy the requirements of section 62(c) and § 1.62-2 (e.g., the arrangement does not require expenses to be substantiated or require amounts in excess of the substantiated expenses to be returned), all amounts paid under the arrangement are treated as paid under a nonaccountable plan, are included in wages, and are subject to withholding and payment of employment taxes when paid.(c)Effective dates. This section generally applies to payments made under reimbursement or other expense allowance arrangements received by an employee on or after July 1, 1990, with respect to expenses paid or incurred on or after July 1, 1990. Paragraph (b)(1)(ii) of this section applies to payments made under reimbursement or other expense allowance arrangements received by an employee on or after January 1, 1991, with respect to expenses paid or incurred on or after January 1, 1991.T.D. 8324, 55 FR 51696, Dec. 17, 1990