Electrocatalytic activity for molecular oxygen reduction on nanoparticles of ruthenium mixed with Nafion(R) in 0.5 M H2SO4 was studied. The electrocatalytic material was synthesized by pyrolysis of Ru-3(CO)(12) in sealed ampoules at 190 degreesC for 3 h. The product of the pyrolysis was characterized by DSC, FTIR, XRD, SEM (EDX), TEM and also electrochemically. X-ray diffraction and TEM studies indicated that the synthesized powder presents a nano-crystalline structure. The electrochemical results obtained by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques show that the oxygen reduction reaction take place by a multi-electronic charge transfer process (n = 4e(-)) with the formation of ca. 4.5% of hydrogen peroxide. A Tafel slope of ca. -100 mV dec(-1) and an exchange current density of 7.9 x 10(-7) mA cm(-2) were deduced. The kinetics of the reduction reaction were evaluated in the potential range of 0.7-0.0 V (SHE) through a simple Damjanovic model. Further interpretation of the analysis of the kinetic results in the light of Anastasijevic's procedure is also presented. (C) 2004 Elsevier B.V. All rights reserved.