Mixtures of RhO x +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°C 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 5–10mol.% Rh composition range. A significant increase in the electrodes’ active area is observed for compositions corresponding to more than 10mol.% RhO x in admixture with Co3O4. Above 10mol.% Rh, the voltammetric curves become more similar to that for RhO x and then RhO x 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 500–800Fg−1 over to 20–60mol.% RhO x 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.