A two-step hydrothermal process was developed to synthesize hydrous 30RuO2–70SnO2 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. 1150Fg−1, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150°C for 2h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500mVs−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 (RuδSn1−δ)O2·xH2O onto partially dissolved SnO2·xH2O and the formation of (RuδSn1−δ)O2·xH2O 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·xH2O, suggest the deposition of RuO2-enriched (RuδSn1−δ)O2·xH2O onto SnO2·xH2O seeds as well as the individual formation of (RuδSn1−δ)O2·xH2O crystallites in the second hydrothermal step.