The acetone-sensitized photolysis of 5-bromouracil (BrU), 1-methyl-5-bromouracil (MeBrU) and 5-bromouridine (BrUd) in deoxygenated aqueous solution in the presence of alcohols was studied on irradiation at 254 nm. BrU is converted to uracil with the release of Br-, as shown by UV analysis and high performance liquid chromatography. The quantum yield of the BrU-) uracil photoconversion, using 0.1 M acetone and approximately 1 M 2-propanol, increases linearly with the inverse square root of the intensity (/254) and approaches values of 15 at pH 13 for 1254=0.06 mW cm -2. Comparable results were obtained for MeBrU and BrUd; the maximum ~ values are 12 and 4 respectively, tit)(-BrU) decreases significantly with decreasing pH and decreasing concentrations of either acetone or 2-propanol (at a given pH). No evidence for the involvement of the triplet state of BrU in the photoreduction was found. ¢)(-BrU) is reduced when 2-propanol is replaced by methanol and significantly reduced in the presence of tert-butanol. To account for the chain reaction in neutral solution, it is suggested that the (CH3)2"COH radical is initially formed by H-atom abstraction by triplet acetone from 2-propanol; propagation occurs by electron transfer to BrU, Br-release and H-atom abstraction by the uracil-5-yl radical from 2-propanol. The enhancement of q~ -BrU) in the alkaline pH range is ascribed to a lower reactivity of the termination reaction with respect to the competing electron transfer from the (CH3)2"CO-radical to BrU.