The electrocatalytic reduction of dioxygen has been studied by cyclic voltammetry at glassy carbon electrodes (GCEs) modified with four different dicobalt diporphyrin complexes in dioxygen-saturated aqueous solutions. The electroreduction of O-2 on modified GCEs is irreversible and diffusion-controlled. Under the catalysis of these metallodiporphyrins, dioxygen reduces to hydrogen peroxide via a two-electron process. The results reveal that the geometric conformation of diporphyrins determines whether dioxygen is reduced by a four-electron process to water or via a two-electron change to hydrogen peroxide. (C) 2003 Elsevier B.V. All rights reserved.