The relationship between the structure-specific capacitance (F g(-1)) of a composite electrode consisting of activated coconut-shell carbon and hydrous ruthenium oxide (RuOx(OH)(y)) has been evaluated by impregnating various amounts of RuOx(OH)(y) into activated carbon that is specially prepared with optimum pore-size distribution. The composite electrode shows an enhanced specific capacitance of 250 F g(-1) in 1 M H2SO4 with 9 wt.% ruthenium incorporated. Chemical and structural characterization of the composites reveals a homogeneous distribution of amorphous RuOx(OH)(y) throughout the porous network of the activated carbon. Electrochemical characterization indicates an almost linear dependence of capacitance on the amount of ruthenium owing to its pseudocapacitive nature. (C) 2004 Elsevier B.V. All rights reserved.