γ-Radiolysis reactions of eight 5-fluorouracil (5-FU) derivatives having sulfonyl group-containing substituents at the 1-position and five 5-fluorouridine (5-FUR) derivatives having thioureido group-containing substituents were studied under the conditions where hydrated electron (eaq−) and hydroxyl radical (HO·) become the principal reactive species. The 5-FU and 5-FUR derivatives were radiolyzed