Hydrous manganese oxide with promising pseudocapacitive behavior was deposited on a carbon substrate at anodic potentials of 0.5-0.95 V vs. saturated calomel electrode (SCE) in 0.25 M Mn(CH3COO)(2) solution at 25degreesC. The effects of the deposition potential on the material characteristics and electrochemical performances of the hydrous manganese oxide prepared were investigated. Porous manganese oxide with higher crystallinity was formed at a lower deposition potential. When the deposition potential was 0.8 V-SCE or higher, the deposited oxide consisted of an inner layer with a laminated structure and a rough outer layer with nodules on the surface. X-ray photoelectron spectroscopy was also carried out to examine the chemical state of the deposited oxide. Analytical results indicated that the oxide was composed of both trivalent and tetravalent manganese oxides at a deposition potential of 0.5 V-SCE. However, the tetravalent manganese oxide became the dominant species in the film deposited at above 0.65 V-SCE. The manganese oxide formed at 0.5 V-SCE exhibited a specific capacitance as high as 240 F/g, as evaluated by cyclic voltammetry (CV) with a potential scan rate of 5 mV/s in 2 M KCl at 25degreesC. Increasing the CV scan rate reduced the specific capacitance. Only about 70% of the capacitance at 5 mV/s could be maintained when the CV scan rate was increased to 100 mV/s, for all the manganese oxide electrodes prepared. Moreover, a high deposition potential gave rise to a low specific capacitance of the manganese oxide formed. (C) 2003 The Electrochemical Society.