I am separated from my soon to be ex and he has fallen behind in his child support from his previous wife. When filing taxes will I be responsible for paying his back child support?

No. You are not responsible for your spouse's child support obligations.

If, however, you are asking about filing a joint federal income tax return (U.S.), it is possible that his past-due child support could be withheld from a tax refund

No. You are not responsible for your spouse's child support obligations.

If, however, you are asking about filing a joint federal income tax return (U.S.), it is possible that his past-due child support could be withheld from a tax refund

Thank you, yes the IRS can withhold a tax refund.