Four cobalt porphyrins were adsorbed on graphite electrodes and used to catalyze the electroreduction of O-2. The two porphyrins without substituent groups in the meso positions of the porphyrin ring operated at the most positive potentials and catalyzed the reduction of O-2 to both H2O2 and H2O, but the H2O did not result from significant reduction of H2O2. The porphyrins containing meso substituents catalyzed only the reduction of O-2 to H2O2. The catalysts that accomplish the four-electron reduction of O-2 are argued to consist of dimeric (or higher oligomeric) forms of the adsorbed porphyrins. The present results and those of two recent related studies(1,2) indicate that the presence of only hydrogen or small alkyl groups in the meso positions of porphyrin rings facilitates the spontaneous formation of van der Waals dimers with greater catalytic activity for the reduction of O-2 by four electrons. Such cobalt porphyrins were also found to be unusually active catalysts for the electro-oxidation of H2O2.