Size-dependent oxygen reduction reaction activity (ORR) and instability of Pt nanoparticles is of great importance in proton exchange membrane fuel cell applications. In this study, the size-dependence of ORR activity on Pt nanoparticles (NPs) was investigated on high-surface-area carbon supported Pt NPs below 5 nm in acidic electrolytes using rotating disk electrode method. The ORR activity was correlated to the estimated surface coverage by OH anion from cyclic voltammogram measurements and the surface composition and electronic structure of Pt NPs, which was studied using X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy. The results revealed a size-independent ORR activity on Pt NPs below 5 nm, which was attributed to similar surface compositions and surface electronic structures of Pt NPs below 5 nm as well as comparable OH anion coverage at the potential where ORR was evaluated. In contrast, the instability of Pt NPs under accelerated potential cycling was found to be strongly dependent on the particle size. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.009202jes] All rights reserved.