The kinetics of oxidation of ascorbic acid and a series of substituted 1,2- and 1,4-dihydroxybenzene compounds (H₂Q) by [Ru(CN)₆]^3– have been investigated in acidic perchlorate media. The inverse dependences of the rate constants on acid concentrations for 2,3-dicyano-1,4-dihydroxybenzene, 4,5-dihydroxybenzene-1,3-disulphonate, and ascorbic acid have been attributed to concurrent rate-determining pathways involving the one-electron oxidations of H₂Q or HQ^– by [Ru(CN)₆]^3– to the corresponding semiquinone or ascorbate radical intermediate. The cross-reaction rate constants have been correlated with the semiquinone or ascorbate reduction potentials in terms of the Marcus relationship to yield a [Ru(CN)₆]^3––[Ru(CN)₆]^4– electron self-exchange rate constant of (1.0 ± 0.8)× 10⁵ dm³ mol^–1 s^–1. This is compared with those for other low-spin d⁵–d⁶ transition-metal complex couples and discussed in terms of the inner-sphere and solvent reorganization energies.