Vitamin B-12 (acquo-cobalamine) adsorbs irreversibly on ordinary pyrolytic graphite (OPG) electrodes from aqueous solutions. The adsorbed vitamin exhibits cyclic voltammetric reversible waves corresponding to Co(II)/Co(I) and Co(III)/(II) couples and its redox chemistry is similar to that found in solution. The OPG/vitamin B-12(ads) electrodes show substantial catalytic activity for the reduction of oxygen. Tn alkaline pH, a Tafel slope of -60 mV/decade and a chemical order in protons of -1 suggest a redox catalysis type of kinetics, governed by the Co(III)/Co(II) couple in the cobalamine. At pH < 10.9, the Tafel slopes change to -120 mV/decade and the order in protons gradually tends to zero, implying that a first electron transfer step becomes rate controlling. The reaction seems to occur via parallel 2-e(-) and 4-e(-) pathways, with the 4-e(-) reduction reaction predominating at more acid pH and at more negative potentials. At high overpotentials, the overall 4-e(-) reduction appears to be promoted by the cobalamine in the Co(I) oxidation state.