My ex husband passed away in oct 2012. I was granted full custody of our son. When I went to e file my taxes it was rejected as to the step mom claiming my son on their joint return taxes. When I called her she informed me she didn't claim my son but my deceased ex husband did. Is this legal or should I fight it?

October means that was 10 months out of the 12 for the year he was paying support.. they get that deduction for the year.

Not that he is gone... so is the support and you will have that deduction this year

What smoothy says is accurate provided the divorce decree did not did not dictate which parent claims the child.

Only the custodial parent can claim the child. It does not count who paid child support. The custodial parent is the parent with whom the child lived for the longer period based on the nights spent. The other parent is the non-custodial parent. Non-custodial parent can claim the child only if the custodial parent signs a release Form 8332.
If noncustodial parent claims the chid, the noncustodial parent gets exemption deduction and child tax credit, but not the head of household status and EITC.

MukatA,

What you say is true EXCEPT when the divorce decree says otherwise.

The divorce decree is a legal document and contract (which both federal appeals courts and federal tax court has ruled) the IRS must abide by.