Nanocrystalline oxides with average chemical composition corresponding to Ru0.8Me0.2O2-x (Me = Fe, Co, Ni) were prepared by the sol-gel approach. All prepared materials were of single-phase character with rutile-type structure. The effect of nanocrystal size and nature of doping cation on electrochemical supercapacitor behavior was studied by means of cyclic voltammetry and electrochemical impedance. The specific capacitance of the Ru-based oxides increases by doping with lower-valence cation from ca. 22 mu F cm(-2) of actual electrode surface area observed for pure RuO2 to 230 mu F cm(-2) in the case of Ni doped material. The improved capacitance behavior of the doped materials is ascribed to improvement of transport properties of the oxide structure enabling easier diffusion of compensating protons. (c) 2007 The Electrochemical Society.