Hydroxyl radicals were generated in the Fenton reaction at pH 4 (Fe2+ + H2O2 - Fe3+ + (OH)-O-. + OH-, k approximate 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 H2O2 in excess and in the absence of O-2, 1,3-DMU consumption is 3.3 mol per mol Fe2+. 1,3-DMUglycol is the major product (2.95 mol per mol Fe2+). Dimers, prominent products of (OH)-O-.-induced reactions in the absence of Fe2+/Fe3+ (Al-Sheikhly, M.; von Sonntag, C. Z Naturforsch. 1983, 31b, 1622) are not formed. Addition of (OH)-O-. 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 Fe3+ that builds up during the reaction or with H2O2 gives rise to a short-chain reaction that is terminated by the reaction of Fe2+ 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 Fe2+ 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 similar to 22% HO2./O-2(.-) in the course of fragmentation reactions involving the C(5)-C(6) bond. Reduction of Fe3+ by O-2(.-) leads to an increase in (OH)-O-. production that is partially offset by a consumption of Fe2+ in its reaction with the peroxyl radicals (formation of organic hydroperoxides, k approximate to 3 x 10(5) L mol(-1) s(-1); value derived by computer simulation).