The catalytic oxidation of 2-mereaptoethanesulfonic acid (2-MESA) was studied on ordinary pyrolytic graphite (OPG) and vitreous carbon electrodes modified with different surface concentrations of the adsorbed cobalt phthalocyanine (CoPc) and with electropolymerized cobalt tetra-aminophthalocyanine (CoTAPc). The electrocatalytic oxidation of 2-MESA at the examined cobalt phthalocyanine modified electrodes is sensitive to the surface concentration of the catalyst, suggesting that the oxidation of 2-MESA is of first-order on the surface concentration of CoPc, for high coverage. They also tend to indicate that it must be the presence of electronic factors in the monolayer levels of the adsorbed cobalt phthalocyanines which are completely different from those present in the tri-dimensional arrays of the active part of the thicker adsorbed or electropolymerized complex deposits.