The high specific capacitance of ruthenium oxide (denoted as RuOx) nanoparticles prepared by a modified sol-gel method with annealing in air for supercapacitors was demonstrated in this work. The specific capacitance of activated carbon (denoted as AC) measured at 5 mA/cm(2) is significantly increased from 26.8 to 38.7 F/g by the adsorption of RuOx nanoparticles with ultrasonic weltering in 1 M NaOH for 30 min. This method is a promising tool in improving the performance of carbon-based double-layer capacitors. The total specific capacitance of a composite composed of 90 wt.% AC and 10 wt.% RuOx measured at 25 mV/s is about 62.8 F/g, which is increased up to ca. 111.7 F/g when RuOx has been previously annealed in air at 200 degreesC for 2 h. The specific capacitance of RuOx nanoparticles was promoted from 470 to 980 F/g by annealing in air at 200 degreesC for 2 h. The nanostructure of RuOx was examined from the transmission electron microscopic (TEM) morphology. (C) 2003 Elsevier B.V. All rights reserved.