Hydrous ruthenium-tin oxides (denoted as (Ru-Sn)O-2.nH(2)O) were synthesized under a mild hydrothermal condition. The mean particles size of pristine RuO(2)nH(2)O smaller than 3 nm, was decreased with the introduction of Sn into the precursor solutions while it is not significantly affected by varying the Sri content. The textual characteristics of (Ru-Sn)O-2.nH(2)O were analyzed through means of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), electron diffraction, X-ray photoelectron spectroscopy (XPS), and thermograv i metric analyses (TGA). From the ideal capacitive behavior of pristine oxides with the Ru content >= 40 at.%, hydrothermal synthesis favored the formation of hydrous oxides with a novel structure providing excellent pathways for electron hopping and proton diffusion/exchange during the charge storage/delivery process. The introduction of tin oxide was demonstrated to successfully promote the utilization of oxyruthenium species, reaching a maximum C-S.Ru of ca. 830 F/g for pristine Ru0.6Sn0.4O2.nH(2)O (measured at 25 mV/s). (c) 2004 Elsevier Ltd. All rights reserved.