Hydrous crystalline RuO2 nanoparticles were prepared successfully via hydrothermal synthesis at 180 degrees C for 24 h. The RuO2 nanoparticles were highly uniform in size and had a mean diameter of 2.6 nm as indicated by high-resolution analysis electron microscopy. A drip coating method was developed to coat these hydrothermally prepared RuO2 nanoparticles (well dispersed in water) onto a Ti substrate to form a RuO2 film with excellent electrochemical performance. The hydrothermally prepared RuO2 powders show a nanocrystalline structure from X-ray diffraction analysis. Due to the smooth proton exchange pathways and the good electronic conductivity of the RuO2 film during the redox transitions, the CV curve showed capacitive-like behavior under a very high scan rate of CV (reaching 2000 mV/s) with specific capacitance around 210 F/g. (C) 2004 The Electrochemical Society.