For carbon-supported Pt-enriched Pt3Au nanoparticles (Pt3Au/C), the effect of oleylamine (OA) concentration during nanoparticle synthesis is investigated. When the OA concentration is increased from 0.12 mM to 1.2 mM, the particle size of Pt3Au gradually decreased from 5.1 nm to 3.3 nm, whereas the crystal structure and surface Pt concentration are not significantly influenced. With higher OA concentration, the oxygen reduction reaction activity of the Pt3Au/C is largely enhanced, which can be explained by the combined effect of increased specific activity and electrochemical surface area of Pt. It is experimentally confirmed using a bulk CO-oxidation technique that smaller particle size (larger OA concentration) leads to decreased OH adsorption strength, which originates from the double layer overlapping effect of closely adjacent nanoparticles. (C) 2015 Elsevier B.V. All rights reserved.