The hydrothermal-electrochemical method has been used for the synthesis of manganese dioxides from acidic MnSO4 solutions. This method produced manganese dioxides of either the beta- or gamma -structure type. At pH 1 and 130 degreesC, pure beta -MnO2 was formed; this was the first time that this structure has been obtained electrochemically. At 113 or 92 degreesC, mixtures of beta- and gamma -MnO2 were obtained, and the beta -MnO2 content decreased with temperature. Increasing the acidity at 92 degreesC led to the formation of pure gamma -MnO2 with lower levels of pyrolusite intergrowth and microtwinning defects compared to gamma -MnO2 prepared by traditional electrochemical syntheses.