The mechanism of the oxygen reduction reaction (orr) was studied in alkaline media at La0.6Ca0.4CoO3-activated carbon electrodes in a channel flow cell (CFC). The electrodes were made by depositing a mixture of the perovskite (La0.6Ca0.4CoO3) powder as the catalyst, Vulcan XC 72 as the carbon, and Triton X-100 as the binder on Ti substrates. The collection efficiency, N, of the double-electrode system in the CFC was evaluated in advance using the ferro-ferricyanide couple. The results obtained for the ori show that the mean number of electrons transferred per O-2 molecule, n(g), remains around 2, while the ratio k(1)/k(2) (the rate constants for direct reduction to OH- and indirect reduction to HO2-) is much smaller than 1, both at nonactivated and perovskite-activated carbon electrodes. This indicates that most of the oxygen was reduced via the two-electron pathway to HO2-. The hydrogen peroxide ions appear to be stable at Vulcan, but are further reduced and/or chemically decomposed in part in the presence of the metal-oxide catalyst. (C) 2000 Elsevier Science Ltd. All rights reserved.