Hydrous ruthenium oxide/carbon black nanocomposites were prepared by impregnation of the carbon blacks by differently aged inorganic RuO2 sols, i.e. of different particle size. Commercial Black Pearls 2000 (R) (BP) and Vulcan (R) XC-72 R (XC) carbon blacks were used. Capacitive properties of BP/RuO2 and XC/RuO2 composites were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in H2SO4 solution. Capacitance values and capacitance distribution through the composite porous layer were found different if high- (BP) and low- (XC) surface-area carbons are used as supports. The aging time (particle size) of Ru oxide sol as well as the concentration of the oxide solid phase in the impregnating medium influenced the capacitive performance of prepared composites. While the capacitance of BP-supported oxide decreases with the aging time, the capacitive ability of XC-supported oxide is promoted with increasing oxide particle size. The increase in concentration of the oxide solid phase in the impregnating medium caused an improvement of charging/discharging characteristics due to pronounced pseudocapacitance contribution of the increasing amount of inserted oxide. The effects of these variables in the impregnation process on the energy storage capabilities of prepared nanocomposites are envisaged as a result of intrinsic way of population of the pores of carbon material by hydrous Ru oxide particle. (C) 2010 Elsevier RV. All rights reserved.