The radiation-induced reduction of 5-bromo-6-hydroxythymine to produce thymine (2) in deaerated aqueous solution was remarkably promoted by the addition of lower-valent transitionmetal salts (K4Fe(CN)6 (3a), CuCl (3b), K2PtCl6 (3c), and FeSO4(3d)). It is suggested that the possible intermediate hydroxythymine-5-yl radical undergoes one-electron reduction by 3a-d to the corresponding anion which eliminates