We have clearly demonstrated reversible cyclic voltammograms for the redox reaction between the Rh(II) and Rh(III) states in rhodium octaethylporphyrin [Rh(OEP)] adsorbed on carbon black in an acidic aqueous solution. The emergence of the reversible wave can be ascribed to the suppression of the undesirable reactions between two molecules of [Rh-II(OEP)] because of its strong interaction with the carbon black. The generated [Rh-II(OEP)] exhibits a potent catalytic O-2 reduction activity.