A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO2 center dot xH(2)O). The procedure involves directly mixing RuCl2 center dot xH(2)O with alkali to form RuO2 center dot xH(2)O in a mortar at room temperature. Transmission electron microscopy (TEM) and N-2 adsorption-desorption measurement indicate that the RuO2 center dot xH(2)O particle is approximately 30-40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO2 center dot xH(2)O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO2 center dot xH(2)O powder was found in an amorphous phase and the maximum capacitance of 655 Fg(-1) was obtained by annealing at 150 degrees C. Higher temperatures (exceeding 175 degrees C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 Fg(-1) at 175 degrees C to 87 Fg(-1) at 400 degrees C. Also, the dependence of electrochemical performance on annealing conditions of RuO2 center dot xH(2)O was investigated by electrical impedance spectroscopy (EIS) study. (C) 2007 Elsevier B.V. All rights reserved.