Carbon-supported PtRu nanoparticles were prepared by different methods that involve the simultaneous chemical reduction of H2PtCl6 and RuCl3 by NaBH4 at room temperature (PtRu-1), by ethanol under reflux (PtRu-2), and by the thermal decomposition of a single-source molecular precursor [(bipy)(3)Ru] (PtCl6) (PtRu-3). Transmission electron microscopy (TEM) examinations show that the mean diameter of the PtRu nanoparticles is lowest for PtRu-1 followed by PtRu-2 and PtRu-3. Measurements of electrocatalytic properties, however, reveal a different trend, namely: PtRu-3 > PtRu-1 > PtRu-2. This is attributed to the formation of a more homogenous alloy nanoparticle system from the thermolysis of the single-source molecular precursor. All three catalysts are more active than commercially available E-TEK (20 wt.%) Pt catalyst. PtRu-3 also displays the highest tolerance to carbon monoxide. Heat treatment of PtRu-1 and PtRu-2 only marginally affects their electrocatalytic performance, whereas the co-reduction of H2PtCl6 and RuCl3 under alkaline conditions has more adverse Outcomes. (c) 2004 Elsevier B.V. All rights reserved.