From the IRS website:
"Question: Are child support payments deductible by the payer or can the payer claim a dependency exemption for the child?
Answer:No, child support payments are neither deductible by the payer nor taxable income to the recipient.
However, the payer of child support may be able to claim the child as a dependent:
- The child is the qualifying child of the custodial parent, and the custodial parent is generally allowed to claim a dependency exemption for the child, if the other tests for claiming the exemption are met. The parent with whom the child lived for the greater part of the year is the custodial parent for income tax purposes.
- The noncustodial parent may claim an exemption for the child if the special rule for a child of divorced or separated parents (or parents who live apart) applies. This requires in part, that the custodial parent sign a Form 8332, Release/Revocation of Release of Claim to Exemption for Child by Custodial Parent, or a substantially similar statement. The noncustodial parent must attach it to his or her return."