Carbon supported Co-Pd nanocomposites with two different surface compositions and various Co/Pd ratios have been synthesized by an ultrasound-assisted polyol process. Sonochemical treatment of palladium acetylacetonate and cobalt acetylacetonate in ethylene glycol in the presence of a carbon support without added surfactant, pH adjuster, or stabilizer has produced 4-7 nm PdxCo nanoparticles dispersed on the carbon support. The absence of additives has led to purer nanoparticles than produced by conventional wet methods. The structures of nanoparticles - pure Pd, Co-Pd alloy, and core-shell - have been characterized by XRD, TEM, EELS, and STEM. Electrocatalysis of oxygen reduction reactions of the materials have been measured using rotating disk electrodes and are compared with pure Pd/C and commercial Pt/C. The Co-Pd mixed phase samples show higher specific activity than pure Pd/C and comparable performance to commercial Pt/C. The Pd4Co core-shell structure shows significantly enhanced catalytic activity. (C) 2011 Elsevier B.V. All rights reserved.