Composites are prepared by deposition of nanoparticles of RuO2 center dot xH(2)O (1-4 nm) on two carbons: microporous carbon (1.3 nm of average micropore size) and mesoporous carbon (11 nm of average mesopore size). Two-preparation procedures are used: (i) procedure A consisting of repetitive impregnations of the carbons with RuCl3 center dot 0.5H(2)O solutions, and (ii) procedure B based on impregnation of the carbons with Ru(acac)(3) vapour. The procedure B leads to supported RuO2 center dot xH(2)O particles that appear more crystalline than those obtained by the procedure A. Specific capacitance and specific surface area of the composites are discussed as functions of the RuO2 content, and different dependences for the composites derived from the two carbons are found. Mesoporous carbon is better support than microporous carbon. Procedure A leads to supported RuO2 center dot xH(2)O particles with higher specific capacitance than the particles deposited by procedure B. (C) 2008 Published by Elsevier Ltd.