The reduction of colloidal MnO 2 by Mn 2+ in aqueous HClO 4 has been studied by a spectrophotometric method. The reaction product is Mn(III). The reaction is of first order in both colloidal MnO 2 and H + , whereas it presents a fractional order (0.58±0.02) in Mn 2+ . The reaction is retarded by addition of NaClO 4 , but is not affected by addition of tert-butanol. The corresponding activation energy is 29.5±1.3 kJ mol −1 . The reaction is catalyzed by Na 4 P 2 O 7 , and the pyrophosphate-catalyzed reaction is of first order in both colloidal MnO 2 and pyrophosphate and of fractional order (0.64±0.01) in Mn 2+ , whereas its rate presents a complex dependence on the concentration of H + . The pyrophosphate-catalyzed reaction is accelerated by addition of both NaClO 4 and tert-butanol. The corresponding activation energy is 49.7±3.0 kJ mol −1 . Mechanisms in agreement with the experimental data are proposed for both the parent and the pyrophosphate-catalyzed reactions.