Manganese dioxide (MnO 2 ) with a-, b-and d-type structures was controllably synthesized by hydrothermal treatment of an acidic solution of KMnO 4 containing different concentrations of ions at 160 C. The effects of metallic cations, H + and anions on the structures and morphologies of MnO 2 were investigated systematically. The experimental results indicated that cations played a significant part in the formation of MnO 2 with different structures. When K + ions were in competition with H + ions in solution, different MnO 2 structures were formed. a-MnO 2 was formed when the amount of K + was higher than the amount of H + , whereas a higher amount of H + was favorable for the growth of b-MnO 2 structures. When the concentration of K + was much greater than that of H + , d-MnO 2 was obtained. Possible formation mechanisms are proposed based on a series of controlled experiments. The electrochemical properties of supercapacitors based on different types of MnO 2 electrodes were investigated in detail. The specific capacitance measured for MnO 2 strongly depended on the crystallographic structure and decreased in the order a-MnO 2 > d-MnO 2 > b-MnO 2 at a current density of 0.5 A g À1 . A specific capacitance of 535 F g À1 was obtained for a-MnO 2 , but was only 155 F g À1 for b-MnO 2 . a-MnO 2 was more suitable for use as the working electrode at high current densities. Both a-and d-MnO 2 had a poor cycling performance after 3000 cycles, whereas b-MnO 2 showed good stability, maintaining a cycling efficiency of 80% after under the same conditions.