This work presents the synthesis and the structural and electrochemical characterization of novel mixed OsxRuy(CO)(n) electrocatalysts for oxygen reduction in 0.5 mol L (1) H2SO4: their monometallic Os-x(CO)(n) and Ru-y(CO)(n) counterparts were synthesized as well, for comparison purposes. The catalysts were obtained by thermolysis of Ru-3(CO)(12) and Os-3(CO)(12) (either alone or mixed) in three organic solvents: 1,2-dichlorobenzene (b.p. 178-180 C), n-nonane (b.p. 150-151 C) and o-xylene (b.p. 143-145 C), under reflux conditions. The products were characterized by FT-IR spectroscopy and scanning electronic microscopy, and their chemical composition obtained by energy-dispersive X-ray spectroscopy. The electrocatalytic activity of the new materials was evaluated by room temperature RDE measurements, using the cyclic and linear sweep voltammetry techniques: all of them are methanol tolerant ORR catalysts, however, the bimetallic Clusters, in general, show more favorable characteristics to perform this reaction than their monometallic analogues. On this basis, the novel catalysts can be considered as potential candidates to be used is cathodes in PEMFCs and DMFCs. (C) 2008 Elsevier B.V. All rights reserved.