Hydroxyl radicals were generated in the Fenton reaction at pH 4 (Fe(2+) + H(2)O(2) --> Fe(3+) + .OH + OH-, k approximately equal to 60 L mol(-1) s(-1)) and by pulse radiolysis (for the determination of kinetic data). They react rapidly with 1,3-dimethyluracil, 1,3-DMU (k = 6 x 10(9) L mol(-1) s(-1)). With H(2)O(2) in excess and in the absence of O(2), 1,3-DMU consumption is 3.3 mol per mol Fe(2+). 1,3-DMUglycol is the major product (2.95 mol per mol Fe(2+)). Dimers, prominent products of .OH-induced reactions in the absence of Fe(2+)/Fe(3+) (Al-Sheikhly, M.; von Sonntag, C. Z. Naturforsch. 1983, 31b, 1622) are not formed. Addition of .OH to the C(5)-C(6) double bond of 1,3-DMU yields reducing C(6)-yl 1 and oxidizing C(5)-yl radicals 2 in a 4:1 ratio. The yield of reducing radicals was determined with tetranitromethane by following the buildup of nitroform anion. Reaction of 1 with Fe(3+) that builds up during the reaction or with H(2)O(2) gives rise to a short-chain reaction that is terminated by the reaction of Fe(2+) with 2, which re-forms 1,3-DMU. In the presence of O(2), 1.1 mol of 1,3-DMU and 0.6 mol of O(2) are consumed per mol Fe(2+) while 0.16 mol of 1,3-DMU-glycol and 0.17 mol of organic hydroperoxides (besides further unidentified products) are formed. In the presence of O(2), 1 and 2 are rapidly converted into the corresponding peroxyl radicals (k = 9.1 x 10(8) L mol(-1) s(-1)). Their bimolecular decay (2k = 1.1 x 10(9) L mol(-1) s(-1)) yields approximately 22% HO(2)./O(2).(-) in the course of fragmentation reactions involving the C(5)-C(6) bond. Reduction of Fe(3+) by O(2).(-) leads to an increase in .OH production that is partially offset by a consumption of Fe(2+) in its reaction with the peroxyl radicals (formation of organic hydroperoxides, k approximately 3 x 10(5) L mol(-1) s(-1); value derived by computer simulation).