A series of carbon-supported Pd nanoparticles catalysts with average sizes ranging from 2.7 to 8.7 nm was synthesized by an aqueous phase reduction method in the presence of citrate and evaluated for the oxygen reduction reaction (ORR). It was found that the average Pd particle size can be tuned and controlled by the citrate to Pd precursor ratio and the effect of citrate concentration on Pd particle sizes was carefully examined. The catalysts were also characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopies, cyclic voltammetry and rotating disc electrode paleography. Measurements of the ORR activities using a rotating disk electrode show a monotonic increase in specific mass activity with increasing Pd dispersion. On the other hand, the specific surface activity displayed a volcano curve with the maximum value between 5.0 and 6.0 nm. The particle size effect could be attributed to a combination of several size-dependent effects, i.e. changes in the distribution of low index planes on the surface, the relative abundance of low coordination sites, and the Pd electronic states. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.