A detailed mechanism for the oxidation of aryl sulfides by peroxymonocarbonate ion in cosolvent/water media is described. Kinetic studies were performed to characterize the transition state, including a Hammett correlation and variation of solvent composition, The results are consistent with a charge-separated transition state relative to the reactants, with an increaser of positive charge on the sulfur following nucleophilic attack-of the sulfide at the electrophilic oxygen of peroxymonocarbonate. In addition, an average solvent isotope effect of 1.5 +/- 0.2 for most aryl sulfide oxidations is consistent with proton transfer in the transition state of the rate-determining step. Activation parameters for oxidation of ethyl phenyl sulfide in tert-butyl alcohol/water are;reported. From the pH dependence of oxidation rates and C-13 NMR equilibrium experiments, the estimated pK(a) of peroxymonocarbonate was found to be similar to 10.6.