In this report, the synthesis and characterization of trimetallic (Pd,Co)@Pt nanoparticles (NPs) with Pt-enriched surfaces are detailed. (Pd,Co)@Pt NPs supported on carbon with different elemental compositions (Pd50Co20Pt30, Pd34Co27Pt39, and Pd21Co34Pt45) are synthesized by sonochemical reactions of Pt(acac)(2), Pd(acac)(2), and Co(acac)(2) in ethylene glycol. The NPs are subsequently characterized by X-ray diffractometry, transmission electron microscopy, and inductively coupled plasma atomic emission spectroscopy to determine their particle size, morphology, and elemental composition. The existence of a Pt-enriched surface on the (Pd,Co)@Pt NPs is demonstrated by line profiles obtained via scanning transmission microscopy energy - dispersive spectroscopy. The NPs are applied to electrocatalysis for oxygen reduction reactions. When compared to a commercial Pt catalyst, the onset potential of the NPs increased by 22 mV, while the specific and mass activities were enhanced by factors of similar to 2.7-4.9 and 4.3-6.3, respectively. The (Pd,Co)@Pt NPs also showed superior stability, as the onset potential was reduced by 11-19 mV after 5000 potential cycles when compared to the 45 mV reduction observed for a commercial Pt catalyst. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.