We examined the durability of a carbon-supported Pt-based Co alloy catalyst (Pt-Co/C) during voltage cycling tests under supersaturated humidity conditions to simulate events such as flooding. The voltage cycling tests were conducted from 0.4V to 1 V for up to 50,000 cycles using 100% relative humidity (RH) hydrogen as the anode gas and 189% RH nitrogen as the cathode gas. We performed cyclic voltammetry (CV) to evaluate the electrochemical surface area (ECSA) and investigated polarization properties to evaluate the activity of Pt-Co/C. The ECSA of Pt-Co/C decreased more slowly than that of a platinum catalyst (Pt/C) during tests (percentages of initial ECSA were 62% and 37% for Pt-Co/C and Pt/C, respectively), either because (1) initial Pt-Co/C was not dispersed unlike Pt/C, or (2) Pt/C exhibited stronger resistance to agglomeration than Pt-Co/C. We concluded that Pt-Co/C exhibited high oxygen reduction reaction (ORR) activity and durability during voltage cycling tests because the mass activity of Pt-Co/C was about twice that of Pt/C throughout the 50,000 cycles. Electrocatalytic specific activities of Pt-Co/C were constant and about twice those of Pt/C for 4000-50,000 cycles, indicating that alloying can increase intrinsic activity for up to 50,000 cycles. (C) 2013 Elsevier Ltd. All rights reserved.