The hydrothermal-electrochemical method has been used for the synthesis of manganese dioxides from acidic MnSO4 or A(2)SO(4)/MnSO4 ( A = Li, Na, K, NH4) solutions. This method produced manganese dioxides of either the beta-, gamma- or alpha- structure types, mixtures of these structure types, alpha/gamma or gamma/beta, or interconnected alpha- and gamma- phases (alpha.gamma). The structure of the material obtained can be controlled by adjusting the parameters of the synthesis: temperature, acidity of the solution, composition of the solution, or applied current density. The gamma-MnO2 materials synthesized are characterized by exceptionally low amounts of microtwinning defects. These materials can be synthesized over a wide range of P-r values, including in the range usually reserved for chemically prepared (chemical manganese dioxide, CMD) materials. Materials exhibiting lower P-r values are favored when the pH or the temperature of the synthesis is decreased. The lowest P-r values were found in materials synthesized at 92degreesC and pH 0. A study of the morphology of the deposits has shown that it can be controlled by changing the experimental conditions. (C) 2003 The Electrochemical Society.