The oxidation of ascorbic acid (H(2)A) by a trans-dioxoruthenium(VI) species, trans-[Ru-VI(tmc)(O)2](2+) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), has been studied in aqueous solutions under argon. The reaction occurs in two phases: trans-[Ru-VI(tmc)(O)(2)](2+) + H(2)A -> trans-[Ru-IV(tmc)(O)(OH2)](2+) + A, trans-[Ru-IV(tmc)(O)(OH2)](2+) + H(2)A -> trans-[Ru-II(tmc)(OH2)(2)](2+) + A. Further reaction involving anation by H(2)A occurs, and the species [Ru-III(tmc)(A(2-))(MeOH)](+) can be isolated upon aerial oxidation of the solution at the end of phase two. The rate laws for both phases are first-order in both Ru-VI and H(2)A, with the second-order rate constants k(2) = (2.58 +/- 0.04) x 10(3) M-1 s(-1) at pH = 1.19 and k(2)' = (1.90 +/- 0.03) M-1 s(-1) at pH = 1.24, T = 298 K and I = 0.1 M for the first and second phase, respectively. Studies on the effects of acidity on k(2) and k(2)' suggest that HA(-) is the kinetically active species. Kinetic studies have also been carried out in D2O, and the deuterium isotope effects for oxidation of HA(-) by Ru-VI and Ru-IV are 5.0 +/- 0.3 and 19.3 +/-2.9, respectively, consistent with a hydrogen atom transfer (HAT) mechanism for both phases. A linear correlation between log(rate constants) for oxidation by Ru-VI and the O-H bond dissociation energies of HA(-) and hydroquinones is obtained, which also supports a HAT mechanism.