The kinetics of the oxidation of hydroquinone (H(2)Q) and its derivatives (H(2)Q-X) by trans-[Ru-VI(tmc)(O)(2)](2+) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) have been studied in aqueous acidic solutions and in acetonitrile. In H2O, the oxidation of H(2)Q has the following stoichiometry: trans-[Ru-VI(tmc)(O)(2)](2+) + H(2)Q -> trans-(Ru-IV(tmc)(O)(OH2)](2+) + Q. The reaction is first order in both Ru-VI and H(2)Q, and parallel pathways involving the oxidation of H(2)Q and HQ(-) are involved. The kinetic isotope effects are k(H2O)/k(D2O) = 4.9 and 1.2 at pH = 1.79 and 4.60, respectively. In CH3CN, the reaction occurs in two steps, the reduction of trans-[Ru-VI(tmc)(O)(2)](2+) by 1 equiv of H(2)Q to trans-[Ru-IV(tmc)(O)(CH3CN)](2+), followed by further reduction by another 1 equiv of H(2)Q to trans[Ru-II(tmc)(CH3CN)(2)](2+). Linear correlations between log(rate constant) at 298.0 K and the O-H bond dissociation energy of H(2)Q-X were obtained for reactions in both H2O and CH3CN, consistent with a H-atom transfer (HAT) mechanism. Plots of log(rate constant) against log(equilibrium constant) were also linear for these HAT reactions.