Carbon-supported bimetallic Au-Pt nanoparticle catalysts with a core-shell structure were prepared by the successive reduction method. UV-vis spectra, TEM, XPS, and XRD techniques were used to characterize the prepared core-shell and the monometallic catalysts. Results of all the physical characterizations showed the continuous growth of the Pt shell on the Au core. The electrocatalytic activities for oxygen reduction were characterized using the rotating disk electrode technique in an acid electrolyte, and were compared with those obtained on a Pt/C catalyst under the same measurement conditions. It was found that these core-shell nanoparticles exhibited the structural characteristics of mainly fcc Au nanocrystals but the electrochemical properties of a Pt surface. The electrocatalytic activities of these core-shell nanoparticles showed a transition from the low activity of Au to the high activity of Pt, and the catalyst with a Au-Pt atomic ratio of 2:1 had a maximized specific mass activity, proving the enhanced Pt utilization with the core-shell structure. (C) 2009 Elsevier Inc. All rights reserved.