This work discusses the electrocatalysis of the oxygen reduction reaction (ORR) in alkaline media on ultra-thin porous coating electrodes formed by Pt and Pt-Co particles dispersed on carbon (Pt/C and Pt-Co/C, 1:1, 1:3, 1:5, 1:8 Pt:Co atomic proportion). XANES (X-ray absorption near edge structure) analysis was carried out in situ to examine the electronic properties of the Pt and Co atoms. X-ray diffraction was employed to estimate the catalyst particle size and to characterize the crystalline structure. The electrochemical techniques considered were cyclic voltammetry and steady state polarization, which were made using the standard rotating ring/disk electrode. Values of the kinetic parameters of the ORR obtained at the lower Pt/C ratios indicated participation of carbon in the catalysis of the reaction, leading to the formation of large amounts of HO2-. At low overpotentials, the catalytic activity per mass of Pt obtained from mass-transport corrected Tafel plots for the 20 wt.% Pt-Co/C materials with lower Co contents are close that of the standard 20 wt.% Pt/C catalyst. (C) 2004 Elsevier Ltd. All rights reserved.