Mixtures of RhOx + Co3O4 have been electrochemically studied by cyclic voltammetry in acid solution as a function of composition. The electrodes were prepared by thermal decomposition at 400 degreesC of mixtures of nitrate precursors. Their electrochemical behavior shows substantial dependence on the electrode's composition. The Co site controls the electrochemical behavior of the system in the 5-10 mol.% Rh composition range. A significant increase in the electrodes' active area is observed for compositions corresponding to more than 10 mol.% RhOx in admixture with Co3O4. Above 10 mol.% Rh, the voltammetric curves become more similar to that for RhOx and then RhOx becomes able to stabilize the Co3O4 in the mixture. Electrodes of this kind have been found to perform as good materials in electrochemical capacitor applications, exhibiting specific capacitances of 500-800 F g(-1) over to 20-60 mol.% Rho, composition range. The large specific capacitance exhibited by this system arises from a combination of the double-layer capacitance and the pseudocapacitance associated with Rh surface redox-type reactions. (C) 2004 Elsevier Ltd. All rights reserved.