The electrocatalytic reduction of dioxygen is investigated by cyclic voltammetry and chronoamperometry at a glassy carbon electrode and a carbon microelectrode adsorbed with cobalt porphyrins in dioxygen-saturated aqueous solutions of various pH. The reduction potential (E(p)) of dioxygen at chemically modified electrodes shows dependence on pH. Electrochemical reduction of dioxygen at monomeric Co-1 or dimeric Co-2-2 modified electrodes establishes a pathway of 2e(-) reduction to form hydrogen peroxide, but it carries out 4e(-) reduction of dioxygen to water at the electrode adsorbed with dimeric Co-2-3 or Co-2-4 in all pH solutions. The electrocatalytic pathway of dioxygen reduction by cofacial biscobalt porphyrins is independent of pH, but the E(p) of dioxygen reduction is dependent on pH. The electrochemical reduction of dioxygen is irreversible and diffusion controlled.