A glassy carbon electrode (GCE) modified with a monolayer of 4-aminothiophenol (4-ATP) and 5,10,15,20-meso-tetraphenylporphyrin cobalt (CoTPP) was prepared and used to examine the electroreduction of oxygen. Cyclic voltammetry showed that CoTPP chemisorbed on carbon via a 4-ATP monolayer (GCE/4-ATP-CoTPP) displayed a catalytic effect similar to that of physisorbed CoTPP on a GCE (GCE/CoTPP). UV-visible spectra recorded in solution and on chemisorbed layers showed that pentacoordination of the cobalt and a cobalt oxidation state +II were required to facilitate the complexation with oxygen. A stable oxygenated intermediate (4-ATP-Co((II))TPP-O(2)) was identified both in solution and on a carbon substrate. Finally, UV-visible reflectance (UVDRS) spectra recorded at different potentials revealed the relation between this oxygenated intermediate and the production of peroxide. Based on these data a catalytic mechanism is proposed. (C) 2010 Elsevier B.V. All rights reserved.