Highly dispersed Pt nanoparticles have been extensively studied for the electrocatalytic oxygen reduction reaction (ORR). Pt bulk and supported-nanoparticle electrodes have exhibited varying degrees of surface structure sensitivity toward the ORR for two main reasons: first, preferential adsorption of supporting electrolyte or water; and second, intrinsic variation of reaction kinetics on different Pt(h k l) surfaces or atomic scale imperfections on the Pt surface (e.g. steps, kinks, edges, and corners). The impact of surface atom coordination on ORR activity is seldom reported because there are few techniques that lend themselves to detailed, in situ assessment of catalyst surface site distribution. Surface active sites on ORR electrocatalysts have been inferred from application of bulk crystal structure data to specific nanoparticle geometries that account for electrocatalytically active surface area, ECA (cm(2)/g(Pt)). This approach fails to capture the wide variety of active sites present on electrocatalyst surfaces under operating conditions, particularly at nanoparticle sizes that span the atomic cluster to nanocrystal transition. In this paper, we apply the techniques developed by Feliu et al. to determine surface site distribution in situ and, for the first time in the field, correlate these observations with ORR mass activity, MA (A/g(Pt)), and surface activity, SA (mu A/cm(Pt)(2)) on Pt nanoparticle catalysts. This approach indicates that the predominant active site available for ORR on nanoparticles in the size range of 1.8-6.9 nm is (1 1 0) or (3 11). This observation is confirmed by using perchloric acid, sulfuric acid, and potassium hydroxide to demonstrate that the supporting electrolyte has little influence on ORR kinetics for these nanoparticles. Such behavior suggests that the Pt nanoparticle surfaces investigated consist of stepped adlayers on (1 1 1) or (1 0 0) facets that eliminate the (1 1 1) terraces historically associated with ORR activity. The predominance of such a stepped surface on Pt ORR electrocatalysts is unexpected and demonstrates the need for in situ characterization of active site distribution. (C) 2013 Elsevier Ltd. All rights reserved.