Electrocatalytic activity for molecular oxygen reduction on nanoparticles of ruthenium mixed with Nafion® in 0.5 M H2SO4 was studied. The electrocatalytic material was synthesized by pyrolysis of Ru3(CO)12 in sealed ampoules at 190 °C 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 mVdec−1 and an exchange current density of 7.9× 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.