The Internal Revenue Service (IRS) lets employees claim a tax deduction of their business expense reimbursements if the expense: was an ordinary and necessary one for their job, is substantiated or proved by the employee, usually with receipts, within a reasonable amount of time, and the employee returned any excess reimbursement to their employer within a reasonable period of time. If any of these conditions are not met, then the reimbursement is considered income and has to be reported as taxable.