Two types of composite materials composed of Pt and WC1-x, nanoparticles supported on multiwalled carbon nanotubes (MWNT) are synthesized and evaluated in terms of their electrochemical properties, especially for the hydrogen oxidation reaction (HOR). The Pt nanoparticles are prepared by reduction of H2PtCl6 with NaBH4, and the WC1-x nanciparticles by a sonochernical method with a W(CO)(6) precursor. One of the composites is synthesized by forming WC1-x nanciparticles on Pt-loaded MWNT and the other by physically mixing Pt-loaded MWNT with WC1-x-loaded MWNT. The sonochemical synthesis of WC1-x on Pt-loaded MWNT forms WC1-x preferentially on Pt nanciparticles, which makes intimate contact between WC1-x and Pt nanoparticles. The cyclic voltammograms of these composite materials show evidences for H+-spill-over from Pt to WC1-x, thereby increasing the electrochemically active surface area (ECA). The composite in which WC1-x is deposited on Pt shows a remarkable increase in ECA probably because the intimate contact between WC1-x and Pt enhances the H+-spill-over. These materials exhibit enhanced HOR characteristics with Pt-specific mass activities about twice that of pure Pt nanoparticles. (c) 2009 Elsevier B.V. All rights reserved.