In this work, novel osmium electrocatalysts for oxygen reduction and hydrogen oxidation in 0.5 M H2SO4, have been developed. The syntheses were performed by thermolysis Of Os-3(CO)(12) and Os-3(CO)(12)/Vulcan(R), in two reaction media, N-2 (in the absence of solvents) and n-octane, in order to evaluate the effect of these parameters on the electrocatalytic activity of the new materials. In the solvent-free pathway, different reaction temperatures (in the 120-320 degrees C range) and times (5, 7 and 10h) were explored; the syntheses in n-octane were done at reflux temperature, for 30 and 72h. The products were characterized structurally by FT-IR spectroscopy, X-ray diffraction and scanning electron microscopy, and electrochemically by room temperature rotating disk electrode measurements, using cyclic and linear sweep voltammetry. Some materials prepared in both reaction media can efficiently perform the hydrogen oxidation and/or oxygen reduction reaction, i.e. those prepared by pyrolysis Of Os-3(CO)(12)/Vulcan(R) in N-2, at 180 degrees C/7h, 320 degrees C/5h, 320 degrees C/7h and 320 degrees C/10h, as well as the materials synthesized in n-octane (from both Os precursors); the latter, in addition, have the important property of being tolerant to carbon monoxide to some extent, in contrast to platinum, which is easily deactivated even by traces of CO. (C) 2007 Elsevier B.V. All rights reserved.