The aim of this work is primarily to rationalize the effect of surface composition on electrocatalytic activity. To investigate this point, we compared two types of nanoparticles with a different surface composition, namely Fe-rich and Pt-Fe mixed surfaces. We synthesized highly dispersed carbon-supported Pt1Fex (x = 1, 2, and 3) nanoparticles with the Fe-rich surface (similar to 2 nm), through a preferential interaction of a capping agent and the metal, i.e., Fe-OOC. The electronic structure and electrocatalytic properties of Pt1Fex nanoparticles with the Fe-rich surface were found to be virtually independent from the Pt/Fe ratio. In contrast, nanoparticles with the Pt-Fe mixed surface, prepared by utilizing the difference of segregation energy, showed a clear dependence of the electronic and electrochemical characteristics on the amount of Pt and Fe, possibly because of the interaction between these two metals on the surface of the electrocatalysts. Compared to Pt, the Pt1Fe2 nanoparticles with the Pt-Fe mixed surface showed the highest enhancement in the activity of the oxygen reduction reaction. This resulted from the development of a more electrochemically stable structure of the Pt-Fe mixed surface. This study demonstrated that the electrocatalytic properties of the Pt-Fe nanoparticles can be tuned using the surface composition rather than the bulk composition. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.