Nanostructured hydrous ruthenium-tin oxides [denoted as (Ru-Sn)O(x)(.)nH(2)O] with variable Sn contents were fabricated by a modified sol-gel process. The mean particle size of pristine (Ru-Sn)O(x)(.)nH(2)O, smaller than 3 nm, was monotonously decreased with increasing the mole fraction of Sn in precursor solutions. The introduction of tin within the RuO(x)(.)nH(2)O matrix with annealing in air between 150 and 250degreesC for 2 h promoted the utilization of electroactive oxyruthenium species (reaching a maximum capacitance of ca. 860 F/g based on RuOx) when the Sn content was lower or equal to 50 atom %. The capacitive performance of (Ru-Sn)O(x)(.)nH(2)O was studied by cyclic voltammetry. The compositions of(Ru-Sn)O(x)(.)nH(2)O oxide were obtained from energy-dispersive spectroscopic analysis. The textural characteristics of these binary oxides were examined by means of transmission electron microscopy, electron diffraction, and X-ray diffraction analysis. (C) 2005 The Electrochemical Society. All rights reserved.