Oxygen reduction characteristics of graphite electrodes modified with aggregated cobalt-porphyrins heat-treated at various temperatures and then impregnated in Nafion polymer were investigated systematically The aggregated cobalt-porphyrin compound was adsorbed on graphite powder and then heat-treated at various temperatures ranging from 200 through 1200 degrees C. The catalysts were evaluated for electroreduction performances of oxygen on modified electrodes in sulfuric acid solutions. The electrocatalytic performances of catalysts as measured in rotating ring-disk electrodes showed that the most effective catalytic activity for oxygen reduction was attained for the aggregated cobalt-porphyrin compounds on graphite powder heat-treated at temperatures between 600 and 800 degrees C. The surface concentration of Co and N as measured by X-ray photoelectron spectroscopy increased as the heat-treatment temperature was increased up to 800 degrees C. The electrochemical performance of pyrolyzed cobalt-porphyrin catalysts changed in parallel with the surface concentration of Co and N. In the temperature range 600-800 degrees C, it appeared that the increased catalytic activity originated from the well-dispersed Co-N-4, moiety or from fragments of the original molecules still retaining the cobalt bound to nitrogen atoms. In the higher temperature region, Co-N-4 bonds were no longer detected, and the presence of cobalt in the metallic states (beta-Co) in the catalysts was confirmed by X-ray diffraction analysis. From the results of the stability tests, the pyrolyzed cobalt porphyrin electrode systems were found to be more stable than the nonheat-treated catalysts.