Electrocatalysis by CoTCP {CoTCP = meso-tetrakis(4-pyridyl)porphynatocobalt(III)} coordinated to four [Ru-3(mu(3)-O)(mu(2)-CH3CO2)(6) (py)(2)](+) complexes in the four-electron reduction of dioxygen, has been unequivocally demonstrated in this work by using two types of electrostatically assembled films of CoTCP and anionic zinc or free-base meso-tetrakis(4-sulfonatophenyl)porphyrins (ZnTPPS or H2TPPS), as well as, by employing different electrode materials. An enhanced electrocatalytic activity has been observed, in spite of the fact that the typical bis-coordination of dioxygen to two cobalt porphyrin sites is precluded in such CoTCP/ZnTPPS or CoTCP/H2TPPS bilayered films. In addition. negligible ring currents have been observed in ring-disk voltammetry measurements, yielding straight Levich and Koutecky-Levich plots, whose slopes approached the theoretical 4e(-) curve in air, or in O-2 saturated solutions (pH 3-5). The use of gold instead of glassy carbon (GC) electrodes has not also significantly perturbed the mechanism. By ruling out any influence from the electrode materials, a well known critical point in the catalysis by cobalt porphyrins, the results have shown that CoTCP is acting as a four-electron transfer catalyst for dioxygen reduction. Also, by excluding the possibility of bis-coordination of dioxygen, it was shown that the electronic and supramolecular effects exerted by the peripheral ruthenium cluster complexes should be triggering the four-electron catalytic activity of the cobalt porphyrin center. (C) 2004 Elsevier Ltd. All rights reserved.