You won't find it writing, in the IRS pubs. The general rule is that a taxpayer must actually make the payment (as well as be legally liable to pay it) to get the deduction. However, there is a recent tax court ruling that seems to say that you can take the deduction even if he was the one that actually paid it under the theory that his paying the expense is just another form of child support and it was your money that paid for it.
