The electroreduction of O-2 at rotated graphite disk electrodes coated with cobalt tetra-pyridylporphyrin was examined at potentials on the rising portion of the current-potential curve, both before and after four Ru(NH3)(5)(2+) groups were co-ordinated to the pendant pyridine ligands. The unruthenated porphyrin provides a two-electron reduction of O-2 with a Tafel slope near 120 mV per decade. The ruthenated porphyrin provides a primarily four-electron reduction with a Tafel slope near 90mV per decade. Some mechanistic implications of the results are discussed including the likelihood that an array of partially reduced and partially oxidized ruthenated porphyrins participates in the catalysis of the reduction of O-2 via a set of parallel reactions.