Pyrimidinyl radicals of various structures (Pyr(center dot)) were generated in aqueous and alcohol-containing solutions by means of pulse radiolysis to determine the rate constants of their repair reactions by different thiols (RSH = cysteamine, 2-mercaptoethanol, cysteine, and penicillamine): Pyr(center dot) + RSH -> PyrH + RS center dot. C5-OH and C6-OH adduct radicals of the pyrimidines react with thiols with k(9) = (1.2-10.0) x 10(6) dm(3) mol(-1) s(-1). Similar repair rate constants were found for uracil- and thymine-derived N1-centered radicals, k(31) = (1.5-6.1) x 10(6) dm(3) mol(-1) s(-1). However, pyrimidine radical anions protonated at their C6 position and C6-uracilyl radicals, with carbonyl groups at their C5 position, react with thiols faster, with k(24) = (0.5-7.6) x 10(7) dm(3) mol(-1) s(-1) and k(14) = (1.4-4.8) x 10(7) dm(3) mol(-1) s(-1), respectively. Quantum chemical calculations, at the B3LYP/6-31G(d, p) and self-consistent reaction field polarizable continuum model level point to the combined effects of the energy gap between interacting molecular orbitals, charge distribution within different pyrimidine-derived radicals, and the coefficients of the atomic orbitals as the possible reasons for the differences in the rate constants of repair.