The reduction of colloidal MnO2 by Mn2+ in aqueous HClO4 has been studied by a spectrophotometric method. The reaction product is Mn(III). The reaction is of first order in both colloidal MnO2 and H+, whereas it presents a fractional order (0.58 +/- 0.02) in Mn2+. The reaction is retarded by addition of NaClO4, but is not affected by addition of tent-butanol. The corresponding activation energy is 29.5 +/- 1.3 kJ mol(-1). The reaction is catalyzed by Na4P2O7, and the pyrophosphate-catalyzed reaction is of first order in both colloidal MnO2 and pyrophosphate and of fractional order (0.64 +/- 0.01) in Mn2+, whereas its rate presents a complex dependence on the concentration of H+. The pyrophosphate-catalyzed reaction is accelerated by addition of both NaClO4 and tert-butanol. The corresponding activation energy is 49.7 +/- 3.0 kJ molt. Mechanisms in agreement with the experimental data are proposed for both the parent and the pyrophosphate-catalyzed reactions.