2′-Deoxy-5-fluorouridine (5-FUdR) derivatives having various types of sulfur-containingsubstituents at the 5′-O-position were synthesized and their γ-radiolyses were studied in aqueous solutions. The γ-radiolysis of compounds having 1,3-dithiol-2-yl and 1,3-dithian-2-yl substituents at the 5′-O-position efficiently gave 5-FUdR, specifically via the attack of hydroxyl radical. On the other hand, the