Hemin, an iron porphyrin, functions as a carriage of dioxygen in mammalian respiration in the ferrous form. It works also as an electrochemical catalyst of the reduction of dioxygen by reproducing the ferrous form from the ferric one. Since the carriage requires stabilization of the dioxygen adduct, the catalytic reduction of dioxygen with the ferric form might hinder the stabilization. This work aims at finding the discrimination between the functions of the carriage and of the catalysis. A hint of the finding lies in the difference between the remarkable catalytic currents in high concentrations of hemin and negligibly small ones in very low concentrations. The catalytic current at the hemin-coated electrode is confirmed to occur at the stoichiometry of two hemin molecules and one dioxygen molecule. When this stoichiometry is applied to the dioxygen adduct of hemin as an intermediate species of the catalysis, high concentrations of dissolved hemin should provide the catalytic current. Spectro-electrochemistry demonstrates that ferrous hemin is not oxidized simply to the ferric form but is converted to other species, e.g. the dioxygen adduct. (C) 2015 Elsevier B.V. All rights reserved.