The influence of Bi- or Ba-containing compounds on the rechargeability of gamma-MnO2 in alkaline electrolytes has been investigated with AA cells containing cylindrical cathodes and flooded cells containing thin-film type cathodes. In addition to the electrochemical evaluation of the cells, the discharged cathodes were analyzed by X-ray diffraction after washing and drying. The incorporation of bismuth or barium into the cathodes was found to improve the cell cyclability, which is partly due to the suppression of electrochemically inactive phases such as birnessite (delta-MnO2) and hausmannite (Mn3O4). A series of chemical oxidation reactions of Mn(OH)(2) with H2O2 in KOH medium and nonredox reactions of Mn(III) acetate with KOH followed by an analysis of the solid and filtrate indicate that the Mn3+ ions, which were in equilibrium with the solid phases containing Mn(III), disproportionated into Mn(II) compounds and Mn(IV) oxides. Reaction mechanisms involving Mn(III) compounds in KOH solution and the role of bismuth or barium on those reactions are discussed. (C) 2003 The Electrochemical Society.