Cobalt tetra(o-aminophenyl)porphyrin (CoTAPP) was electropolymerized from an acetonitrile solution containing 0.1 M (CoTAPP), at a vitreous carbon electrode, leading to a polymer film, poly(CoTAPP). UV-visible differential reflectance spectra (UVDRS) were recorded respectively. during the electropolymerization process and during the oxygen reduction reaction (orr) at a poly(CoTAPP) electrode. Comparison between the UV-visible spectra of the poly(CoTAPP) electrode, recorded under N-2 and under O-2 in a 0.5 M H2SO4 solution, shows dearly the band associated with the charge-transfer process between the central metal ion and the complex ligand. The initial reduced state corresponding to the active state for the oxygen reduction reaction was confirmed to be Co(II) species.