26 U.S.C. § 681: Limitation on charitable deduction

In computing the deduction allowable under section 642(c) to a trust, no amount otherwise allowable under section 642(c) as a deduction shall be allowed as a deduction with respect to income of the taxable year which is allocable to its unrelated business income for such year. For purposes of the preceding sentence, the term “unrelated business income” means an amount equal to the amount which, if such trust were exempt from tax under section 501(a) by reason of section 501(c)(3), would be computed as its unrelated business taxable income under section 512 (relating to income derived from certain business activities and from certain property acquired with borrowed funds). For disallowance of certain charitable, etc., deductions otherwise allowable under section 642(c), see sections 508(d) and 4948(c)(4).