Ruthenium/carbon composite materials are prepared. by impregnating ruthenium(III) acetylacetonate into a mesoporous carbon (average pore diameter = 12 mn, pore volume = 3.6 cm(3) g(-1)) and then heat treatment at 320 degreesC for 2 h under an argon atmosphere. The metallic ruthenium nanoparticles are converted to pseudo-capacitive hydrous ruthenium oxide by electrochemical oxidation at 0.75 V (versus SCE) for 2 h in 2.0 M H2SO4. The specific capacitance of the composite electrodes, which is the sum of the double-layer capacitance of mesoporous carbon and the pseudo-capacitance of hydrous ruthenium oxide, reaches 243 F g(-1) with heavy loading. As the loading is increased, however, the degree of ruthenium utilization for a pseudo-capacitor becomes poorer, presumably due to a limited conversion to the hydrous oxide form. The rate capability of composite electrodes also decreases with increase in ruthenium loading, due to an increase in both the equivalent series resistance (ESR) and the overall capacitance value.The ESR enlargement is caused mainly an increase in the electrolyte resistance within pores which, in turn, results from a pore narrowing with ruthenium loading Hindered ionic motion in narrowed pores can explain this feature. An increase in the RC time constant with ruthenium loading is further verified by ac impedance measurements. (C) 2003 Elsevier Science B.V. All tights reserved.