The electrocatalytic activity of cytochrome c 3 for the reduction of molecular oxygen was characterized from the studies of the adsorption of cytochrome c 3 and the co-adsorption of cytochrome c 3 with cytochrome c on the mercury electrode by the a.c. polarographic technique. The adsorption of cytochrome c 3 on the mercury electrode is irreversible and is diffusion-controlled. The maximum amount of cytochrome c 3 adsorbed was 0.92 · 10 −11 mol · cm −2 at −0.90 V. The amount of cytochrome c 3 in the mixed adsorbed layer with cytochrome c was determined from the differential capacitance measurement. It was shown that the fractional coverage of cytochrome c 3 can be estimated from its bulk concentration and the diffusion coefficient (1.05 · 10 −6 cm 2 · s −1). Cytochrome c 3 catalyzes the electrochemical reduction of molecular oxygen from the two-electron pathways via hydrogen peroxide to the four-electron pathway at the mercury electrode in neutral phosphate buffer solution. The catalytic activity varies with the bulk concentration of cytochrome c 3. The highest catalytic activity for the oxygen reduction (no hydrogen peroxide formation) is attained when one-half of the mercury electrode surface is covered by cytochrome c 3. The addition of cytochrome c or bovine serum albumin to the cytochrome c 3 solution inhibits the catalytic activity of cytochrome c 3. The reversible polarographic behavior of cytochrome c 3 through the mixed adsorbed layer of cytochrome c 3 and cytochrome c was also investigated.