N,N'-di-8-Quinolyltrimethylenediamine cobalt [Co(dqpr)] and N,N'-di-8-quinolylphenylenediamine cobalt [Co(dqph)] were synthesized and tested as catalysts for electro-reduction of oxygen when immobilized on graphite electrodes. For modification on graphite, the complexes were mixed with graphite powder and heat-treated in Ar atmosphere at 400-800 degrees C, then immobilized on graphite electrode with Nafion solution. The performance was improved significantly by the heat treatment, and it was assumed to be due to the formation of stabilized CoN4 chelate structure still remaining on the graphite surface after pyrolysis. The polarization measurements revealed that the performance of the electrodes modified with the complexes was parallel to that of 5% platinum dispersed on carbon. Durability tests of the heat-treated complex at 80 degrees C in acidic aqueous solutions showed relatively good catalytic performances as oxygen reduction catalysts, as compared with platinum. The potential usage of the complexes was demonstrated as the cathode catalysts in polymer electrolyte fuel cells in combination with fewer amounts of platinum, where a remarkable concerted effect is observed between platinum sites and CoN4 centers dispersed on carbon substrate.