The thiosulfatolysis and sulfitolysis of pentathionate were studied in the pH ranges 4.50-8.0 and 4.0-5.0 at T = (25.0 ± 0.1)°C by HPLC monitoring of the time-dependence and distribution of different sulfur species. The two systems are first order with respect to both reactants. In contrast to tetrathionate, pentathionate and higher polythionates contain inequivalent divalent sulfur atoms; therefore, the complexity of the mechanism increases. The selective heterolytic cleavages of [a]