Nanostructured manganese dioxide has been successfully prepared by a sonochemical method from an aqueous solution of potassium bromate and manganese sulfate. Changing the proportions of reagents leads either to γ- or layered structures of MnO2. The capacitive characteristics of the samples were systematically investigated in aqueous electrolytes through means of cyclic voltammetry and chronopotentiometry methods. The electrochemical properties of MnO2 were strongly affected by the pH of electrolyte employed and this material exhibited ideally capacitive behavior in 0.5M aqueous Na2SO4 solution. A maximum specific capacitance of 344Fg−1 was obtained for the layered structure determined via cyclic voltammetry at a scan rate of 5mVs−1 in 0.5M aqueous Na2SO4 solution at pH 3.3. Excellent electrochemical reversibility of the materials was also demonstrated. Layered structure MnO2 showed higher energy density at high power density than the γ-structure material. Impedance spectroscopy studies revealed that charge transfer resistance of the γ-structure oxide has higher value than that of the layered structure.