Originally Posted by HighFlight

Bonuses are taxed in either of two methods; included as regular salary (which will cause tax fluctuations such as you mention), or as a flat 25% rate, separate from salary. (This all on the federal level; other items come out at the same rate no matter how it is taxed federally, such as state, union dues, SS, etc.)

Also, dependents have no affect on a flat-tax-rate bonus; it's 25% whether you have 0 or 10. One can adjust the withholding on their salary (making it less) to compensate for the higher withholding on a bonus, however.

It all depends on how the company decides to handle the bonus; flat rate or as part of income.