We report on the polymeric surfactant-assisted synthesis and characterization of highly dispersed, uniformly alloyed Cu-rich Pt-Cu bimetallic nanoparticles with Cu contents up to 75 atom % Cu for use as an oxygen reduction reaction (ORR) electrocatalyst at polymer electrolyte membrane fuel cells. Base-metal-rich Pt alloy particles are generally very difficult to prepare as a well-alloyed single-phase material with high dispersion using conventional liquid impregnation/reductive annealing routes. A comparison between the characteristics of Pt-Cu alloy particle obtained from a poly(dimethyl-diallyl ammonium) chloride (PDDA) assisted polyol process and a conventional impregnation method shows that the polyol process is able to form single-phase nanoparticles with a very narrow particle size at temperatures below 200 degrees C. We further investigated the electrochemical behavior of Cu-rich electrocatalysts. We demonstrate the formation of highly active ORR Pt-Cu catalyst phases from a PtCu3 precursor by selective electrochemical dissolution (dealloying) of Cu. The dealloyed catalyst yields ORR surface-area-specific activities rivaling those of the Pt-rich state-of-the-art Pt3Co electrocatalysts. Based on the severe depletion in Cu near the surface combined with moderate surface area increases, we propose geometric rather than electronic or surface-area effects as the origin of the observed activity enhancement.