A series of Pt modified Pd/C catalysts (Pt/Pd/C) with different Pt/Pd molar ratio (Pt:Pd = 1:4, 1:2 and 1:1) are synthesized by a chemical reduction method for oxygen reduction reaction (ORR). X-ray diffraction (XRD), transmission electron microscope (TEM) and cyclic voltammetry (CV) measurements confirm that Pt is deposited on the Pd nanoparticles and the Pt/Pd/C catalysts have a Pd-core@Pt-shell structure. In the half cell testing, the catalytic ORR activity of Pt-1/Pd-2/C and Pt-1/Pd-4/C are superior to commercial Pt/C. Moreover, the electrochemical durability to potential cycling of Pt-1/Pd-2/C and Pt-1/Pd-1/C catalysts is better than Pt/C catalysts. The improved durability is believed to be associated with the dissolution of Pd and the corresponding structure transformation from core-shell structure to Pt-Pd alloy with Pt rich surface. The structure change is confirmed by TEM, CV, XRD and X-ray photoelectron spectroscopy (XPS). In addition, the electrochemical active surface area (ECSA) loss and polarization behavior in the single cell testing also suggest that the Pt/Pd/C show a much better durability than Pt/C catalysts. Similarly, the dissolution of Pd is observed by CV and scanning electron microscope-energy dispersive X-ray spectra (SEM-EDX). The Pt/Pd/C electrocatalysts have high ORR activity and this high activity can be further improved during potential cycling, that is, the activity and durability can be obtained simultaneously. This Pd-core@Pt-shell structure allows for the development of highly active and durable ORR electrocatalysts, with potential for the application in proton exchange membrane fuel cells (PEMPCs). (C) 2012 Published by Elsevier B.V.