The two-electron reduction/oxidation reactions of manganese oxide in alkaline electrolytes have been investigated in the presence and absence of bismuth oxide by both galvanostatic discharge/charge and linear sweep voltammetry followed by examining the products with X-ray diffraction. Although the presence of bismuth helps the formation of birnessite MnO2, it could not completely prevent the formation of Mn3O4. However, the formation of Mn3O4 could be suppressed by controlling the scan rate during linear sweep voltammetry and/or the microstructure of the electrode. Mn3O4 appears to be formed by a chemical reaction between dissolved Mn(II) and Mn(III) complexes (intermediates) that have diffused away from the conductive path (graphite). Both a faster scan rate (discharge/charge rate) and an intimate mixing of the manganese oxide with graphite achieved by ballmilling suppress the formation of Mn3O4 independent of the presence of bismuth.