Dipping of gamma-MnO2 in water solution was shown to result in alteration of both solution and solid phase (degree of oxidation) caused by disproportionation reactions in this system. Presence of Mn2+, Cu2+, and Cd2+ ions do not influence on this reaction, whereas Co2+ ion inhibit it. Manganese concentration in solution is controlled by its pH and degree of manganese dioxide oxidation.