A comparative study of the electrochemical stability of Pt25Cu75 and Pt20Cu20Co60 alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950 C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts. Oxygen reduction reaction (ORR) activity and catalyst stability tests were then performed for both Pt25Cu75 and Pt20Cu20C60 alloy catalysts in membrane electrode assemblies (MEA). ORR activity data, taken both prior to and at the conclusion of 30,000 voltage cycles from 0.5 to 1.0 V vs. reversible hydrogen electrode (RHE), revealed that both Pt25Cu75 and Pt20Cu20Co60 were able to retain both their mass and Pt surface area-based activity advantage relative to Pt/C [R.Srivastava, R Mani, N. Hahn, P. Strasser, Angew. Chem. Int. Ed. 46 (2007), 8988; R Mani, R. Srivastava, R Strasserj. Phys. Chem. C 112 (2008), 2770; S. Koh, R Strasser, J. Am. Chem. Soc. 129 (2007), 12624]. Further analysis revealed that the Pt surface area-based activity, measured at 0.9 V vs. RHE, of commercially available Pt catalysts, as well as that for both Pt25Cu75 and Pt20Cu20Co60 increased on the order of tens of mu Acm(Pt)(-2) per 1000 voltage cycles. This increase in specific activity combined with a reduced ECSA loss resulted in a negligible change for the Pt mass-based activity of Pt25Cu75 alloys annealed at 950 C. (c) 2008 Elsevier B.V. All rights reserved.