Your divorce decree is legally binding on you and your ex-wife. It is not legally binding on the IRS. If your divorce decree took effect after 2008, then in order to claim one of the children on your tax return you have to have a Form 8332 signed by the custodial parent, your ex-wife. The divorce decree should have required her to provide this to you. If you do not have a Form 8332 signed by your ex-wife, then you cannot claim the child if you are not the custodial parent.
