Divorced or separated parents must use special rules to decide whether they or their former spouse can claim the exemption for their child when they file their tax returns. The special rules for divorced or separated parents apply only if:
1. The parents are divorced or legally separated under a decree of divorce or separate maintenance, are separated under a written separation agreement, or lived apart at all times during the last six months of the year, and . . .2. One or both parents provide more than half the child's total support for the calendar year, and . . .
3. One or both parents have custody of the child for more than half the calendar year.
Although there are some exceptions, the parent who has custody of the child for most of the year is usually treated as the parent who provides more than half the child's support. It does not matter whether the parent with custody has been actually providing more than half of the child's support.
A noncustodial parent may claim the exemption for the child if either (1) The custodial parent signs a written declaration that he or she will not claim the exemption for the child and the noncustodial parent attaches this written declaration to his or her return, or (2) A decree or agreement executed before 1985 provides that the noncustodial parent is entitled to the exemption and he or she provides at least $600 for the child's support during the year, unless the pre-1985 decree or agreement is modified after 1984 to specify that this provision will not apply.
More information for divorced or separated individuals can be found in IRS Publication 504.