Under current law, the IRS specifies that only the custodial parent is supposed to claim the child on her taxes. While some exceptions and mechanisms exist for transferring the child from one party's return to another in order to obtain the most advantageous tax treatment possible, these generally require a court's permission or the custodial parent's permission. Sometimes, though, you file your taxes only to learn that your ex claimed the child first.
The Default Position
In most cases, the child support guidelines assume that the custodial parent is the one who will be receiving the benefit of claiming the child on her taxes. This assumption is built into the guidelines; as such, the support you're receiving as a custodial parent is based -- at least in part -- upon the idea that you're going to be getting a tax benefit by virtue of the responsibility you bear with respect to caring for the child. In other cases, such as those where the parties split custody evenly, there is no clear custodial parent. In these situations, the court will generally alternate the right to claim the child between the parties from year to year.
How It Can Be Altered
Although the guidelines usually presume you'll be getting all the deductions and tax benefits for your child, you can agree to release the child from your taxes even if a court doesn't order it. Sometimes, due to the different incomes of the parties, your ex might get more of a benefit from claiming the child than you would. In this case, you can "sell" the tax dependency exemption to your ex by having him pay you what you would have gotten had you claimed the child in exchange for you signing IRS Form 8832, transferring the exemption to him. Parents who do this see it as a way of helping the ex-spouse out without losing any money themselves. Others may do it to recover arrears faster.
What To Do
If you don't agree to release the exemption, it can be a nasty surprise when your ex claims it on his own. You may not know this until you've already filed your return and the IRS sends you a letter saying that you owe them money or you receive little or no tax return. When this happens, you should get a copy of your child custody order and your child support order and attach both to the tax return you originally submitted, along with a letter explaining that you, not your ex, is entitled to claim the dependency exemption for the child. Do the same on your state return. The IRS and your home state's department of revenue will make the appropriate adjustments.
If You Do Nothing
Getting your return straightened out isn't automatic. If you do not bring it to the attention of the tax authorities that you, not your ex, are entitled to claim the child, neither your return nor your ex's return will be adjusted. You're going to have to be your own advocate and ensure that your rights are protected.