Telling your friend there's a job at your office she'd be perfect for might earn you extra money. Some companies reward employees who recruit job applicants with an employee-referral bonus. If the bonus is cash, you pay tax on it. If you receive a non-cash bonus, it's possible that it's tax-free. No bonus is ever taxable until your employer pays you, even if you know it's on the way.
On Your W2
The IRS treats cash bonuses no different than wages. For example, if you get $300 for finding someone to fill a key company position, that $300 should show up on your W2 at the end of the year. Your employer has to take out withholding for income taxes, Social Security and so on just as she does from your regular paycheck. Depending on how your employer calculates bonus withholding, the withholding may be higher than your regular rate.
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Payment in Kind
If your bonus is a gift with a clear cash value -- such as a set of golf clubs, gift certificate or a ticket to the big game -- it's taxable. Like cash, it should show up on your W2. Something that doesn't have a cash value, such as a "top employee" parking spot, isn't taxable. "De minimis" benefits -- those that are so small they're inconsequential -- aren't taxable. If your employer takes you out to dinner to say thanks, the tax impact might depend on how expensive the meal is.