A two-step hydrothermal process was developed to synthesize hydrous 30RuO(2)-70SnO(2) composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol-gel procedure. A very high specific capacitance of RuO2 (C-S.Ru) ca. 1150 F g(-1), was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150 degrees C for 2 h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500 mV s(-1)), demonstrating its excellent power property. From Raman, UV-vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (Ru delta Sn1-delta)O(2)(.)xH(2)O onto partially dissolved SnO2-xH(2)O and the formation of (Ru8Sn1-delta)O(2)(.)xH(2)O crystallites in the second step. This effect significantly promotes the utilization of RuO2 (i.e., very high C-S.Ru). The excellent capacitive performances, very similar to that of RuO2.xH(2)O, suggest the deposition of RuO2-enriched (Ru delta Sn1-delta)O-2-xH(2)O onto SnO2-xH(2)O seeds as well as the individual formation of (Ru delta Sn1-delta)O(2)(.)xH(2)O crystallites in the second hydrothermal step. (c) 2007 Elsevier Ltd. All rights reserved.