Co3O4 + RuO2 electrodes supported on Ni were prepared by thermal decomposition of mixtures of the nitrates at 400 degrees C in the composition range from 0 to 20 mole percent RuO2. The surface morphology was observed on the scanning electron microscope, while the surface composition was determined by x-ray photoelectron spectroscopy. Electrochemical characterization in NaOH solutions included voltammetric curves at different pH and different potential scan rates, as well as with different negative potential limits and different times of cathodic polarization Electrochemical and nonelectrochemical data converge to indicate low porosity and low surface area at low RuO2 contents, while the opposite is the case at high RuO2 concentrations. The surface is remarkably enriched with RuO2. Cathodic polarization degrades the spinel structure. There is some evidence that RuO2 may retard the reduction of Co3O4.