Palladium gold alloy powder (PdAu2) was synthesized by reduction of the [Pd(NH3)(4)][AuCl4](2) complex with H-2 at 90 degrees C, and then was additionally heated in a C2H4 (PdAu2-C-x) or H-2 flow (PdAu2-H-2) at 300 degrees C. The samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and by a set of electrochemical methods. The Pd:Au ratio on the surface of the PdAu2 alloy suffers marginal changes after heating under C2H4 atmosphere, and decreases as a result of heating in a H-2 flow. Hydrogen oxidation reaction (HOR) over the catalysts was studied using rotating disk electrode at 25 degrees C in 0.1 M HClO4 in pure H-2 and in the presence of 1100 ppm of CO. Heat-treatment of the PdAu2 alloy under H-2 does not improve its HOR activity and CO tolerance (normalized to Pd and total metal surface area), while the PdAu2-C-x catalyst exhibits substantially enhanced catalytic performance in the HOR both in the absence and in the presence of CO as compared to the parent PdAu2 alloy. The enhanced activity of the C2H4-treated PdAu2 alloy is explained by the promoting action of carbon atoms incorporated into the alloy structure during treatment under C2H4 atmosphere. This supposition is approved by the fact that mild oxidative treatment of the PdAu2-C-x sample degrades notably its catalytic activity. (C) 2012 Elsevier Ltd. All rights reserved.