Hydrous ruthenium dioxide (RuO2 center dot xH(2)O) prepared in a modified sol-gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO2 center dot xH(2)O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO2 center dot xH(2)O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO2 center dot xH(2)O are attributable to the high-pressured, water-enriched surroundings which restrain coalescence of RuO2 center dot xH(2)O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO2 center dot xH(2)O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO2 center dot xH(2)O with hydrothermal annealing at 225 degrees C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 Fg(-1). (c) 2008 Elsevier Ltd. All rights reserved.