The kinetics of electron-transfer reactions between permanganate ion and ethyl and aryl methyl ketones have been studied in aqueous MeCO2H acid medium in the presence of HClO4 at different temperatures. For ethyl methyl ketone and XC6H4COMe (X = p-Cl, p-Br or p-NO2) the reaction obeys the rate law −d[MnO4−]/dt = (kKe[H+][MnO4−][RCO Me])/(1 + Ke[H+][RCOMe]). But the oxidations of XC6H4COMe (X = p-Me and p-OMe) follow the rate equation −d[MnO4−]/dt = k3[H+][MnO4−][RCOMe]. The reaction involves a fast pre-equilibrium with intermediate formation of a permanganate ester before the two-electron transfer, rate-determining, step. A number of thermodynamic parameters have been evaluated.