Pd clusters were formed on highly dispersed Au nanoparticles (similar to 3.5 nm in diameter) using a seed-mediated growth process. The structural information and electrocatalytic activities of these Pd clusters on Au nanoparticles were confirmed by high-resolution-transmission-electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The resulting nanoparticles, which had a uniform size (<5 nm in diameter), were highly dispersed on carbon particles, and Pd clusters (<0.44 nin in size, <2 atomic layers) were formed selectively on Au nanoparticles. XPS results show that the Pd 3d(5/2) peak shifted to lower binding energies and that the amount of surface oxide decreased as the Pd content was decreased on the Au nanoparticles. In formic acid electro-oxidation, these Pd clusters exhibit enhanced electrocatalytic activity relative to that of carbon-supported Pd nanoparticles. These results may be due to the modified electronic and geometric structure of the Pd clusters on the Au nanoparticle substrate. (C) 2010 Elsevier Ltd. All rights reserved.