We have previously reported that the oxidation of SO32- to SO42- by a trans-dioxoruthenium(VI) complex, [Ru-VI(TMC)(O)(2))](2+) (Ru-VI; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazcyclotetradecane) in aqueous solutions occurs via an O-atom transfer mechanism. In this work, we have reinvestigated the effects of the pH on the oxidation of S-IV by Ru-VI in more detail in order to obtain kinetic data for the HSO3- pathway. The HSO3- pathway exhibits a deuterium isotope effect of 17.4, which indicates that O-H bond breaking occurs in the rate-limiting step. Density functional theory calculations have been performed that suggest that the oxidation of HSO3- by Ru-VI may occur via a concerted or stepwise proton-coupled O-atom transfer mechanism.