MnO2 films electrodeposited on carbon fibers were subjected to hydrothermal treatment in deionized water and a K2SO4 solution. The effects of hydrothermal treatment on the structures and capacitive properties of the films were systematically evaluated. Compared to the as-deposited and K2SO4-treated films, the water-treated film exhibits a much lower capacitance at 2 mV s(-1) due to the phase transformation from ramsdellite to manganite during the hydrothermal treatment. The K2SO4-treated film maintains the ramsdellite structure although the average oxidation state of Mn in the film is decreased to some extent. At the charge-discharge current density of 1C, the K2SO4-treated and as-deposited films show specific capacitances of 281.9 and 274.7 Fg(-1), respectively. But the K2SO4-treated film displays a capacitance of 135 Fg(-1) at 15C and a good high-rate capability which is more than 3 times as high as that of the as-deposited one. These results and the related kinetic analysis suggest that the suitable ratio of Mn4+ to Mn3+ plays an important role in determination of the high-rate capability of the film. (C) 2017 Elsevier Ltd. All rights reserved.