Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four electrodes with average platinum particle sizes of 1.9, 3.2, 7.1 and 12.7 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent decay, when subjected to identical potential cycles. The decay rates of Pt electrochemical surface area (ECA) and mass activity (i(m)) were significantly greater for 1.9 and 3.2 nm Pt-on-carbon catalysts (Pt-C) compared to 7.1 nm Pt-C, which was stable for 10,000 potential cycles. As expected, the performance decay rate of the electrodes with the smallest Pt particle size was the highest and that of the largest Pt particle size was lowest. However, the initial performance of the largest Pt particle size electrode was significantly lower. Thus, a Pt particle size was identified that balanced performance and durability. The relative impact of operational conditions, such as relative humidity, cell temperature and upper potential limit on 3.2 nm Pt electrodes was also evaluated. Highest decay rates were found when the cathode was subjected to a higher upper potential limit. The decay was attributed to a combination of Pt dissolution, particle growth and carbon support corrosion. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.081111jes] All rights reserved.