The MnOOH product forms a solid solution with the reactant,giving rise to the characteristic sloping discharge curve.1 Of the many structural forms of MnO2 which exist, only the gamma form functions well as an alkaline cathode because its surface is not prone to become blocked by the reaction product. In forming MnOOH, the cathode expands about 17% in volume. MnOOH can undergo some undesirable chemical reactions as well. In the presence of zincate ion, MnOOH, through its equilibrium with soluble Mn(III), can form the complex compound hetaerolite, ZnMn2O4. Although electroactive, hetaerolite is not as easily discharged as MnOOH, hence the cell impedance increases. In addition, the MnOOH/MnO2 solid solution can undergo recrystallization into a less active form, resulting in a noticeable loss in cell voltage under certain very slow discharge conditions.