This work discusses the electrocatalysis of the oxygen reduction reaction (ORR) in acid medium on ultra-thin coating electrodes formed by Pt-Ni nanoparticles dispersed on a carbon powder (Pt-Ni/C, 90:10, 75:25 and 60:40, Pt:Ni atomic proportion). The catalysts have been synthesized by a microemulsion method, using Brij((R))30 as surfactant. X-ray diffraction and transmission electron microscopy were employed to estimate the catalyst particle size and to characterize the crystalline structure. This technique indicated that the synthesized catalyst particles present a nano-crystalline structure. The electrochemical techniques considered were cyclic voltammetry and steady-state polarization curves, obtained utilizing the rotating ring-disk electrode (RRDE) technique. These results showed that the ORR takes place by a multi-electronic charge transfer process following a four-electron mechanism. The kinetics of the ORR was evaluated using Tafel diagrams. It is seen that the Pt activity for ORR is enhanced with the increase of the nickel amount on the platinum alloys. (c) 2006 Elsevier B.V. All rights reserved.