Originally Posted by roberton

Unless there's a special exception somewhere for cash back from credit cards, then you technically have taxable income equal to the cash back on purchases reimbursed by your employer. The reason that cash back on a credit card isn't usually taxable is that it's considered to be a discount on your purchase (just as if you paid 2% less). So the cash back itself isn't taxable, but it means that your reimbursement exceeds your expenses (by the amount of the cash back) and that amount is taxable.

That said, I'd be virtually certain that the IRS won't go after this (unless the cash back amounts get to be a lot bigger than they are now -- but if you get to the point where cash back on reimbursed purchases is 25% or 30% of your salary, then I'd start worrying).