The oxidation of methanol and sodium borohydride (NaBH4) on supported colloidal Os and Os-alloys (i.e. Os-Sn, Os-Mo and Os-V) have been studied by voltammetry and chrono-techniques (i.e., potentiometry, amperometry). The electrolytes were: 0.5 M H2SO4 for methanol oxidation and 2 M NaOH (without and with 1.5 x 10(3) M thiourea (TU)) for borohydride oxidation, respectively. Voltammetry results showed that the investigated Os-based materials were catalytically inactive with respect to methanol oxidation, contradicting therefore, the theoretical suggestion by Kua and Goddard [J Am Chem Soc 1999; 121:10928-41] indicating electrocatalytic activity for Os toward methanol oxidation. Regarding the oxidation of borohydride, for direct borohydride fuel cells, preliminary results showed a voltammetric wave, with oxidation potentials between 0.1 and 0.3 V vs. Ag/AgCl, KClstd, which were attributed to borohydride and TU oxidation. Chronopotentiometry results of Os-alloys showed a more positive operating potentials compare to colloidal Os catalysts. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.