The understanding of the degradation mechanisms of electrocatalysts is very important for developing durable electrocatalysts for polymer electrolyte membrane (PEM) fuel cells. The degradation of Pt/C electrocatalysts under potential-static holding conditions (at 1.2V and 1.4V vs. RHE) and potential step conditions with the upper potential of 1.4V for 150s and lower potential limits (0.85V and 0.60V) for 30s in each period [denoted as Pstep(1.4V_150s-0.85V_30s) and Pstep(1.4V_150s-0.60V_30s), respectively] were investigated. The electrocatalysts and support were characterized with electrochemical voltammetry, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Pt/C degrades much faster under Pstep conditions than that under potential-static holding conditions. Pt/C degrades under the Pstep(1.4V_150s-0.85V30s) condition mainly through the coalescence process of Pt nanoparticles due to the corrosion of carbon support, which is similar to that under the conditions of 1.2V-and 1.4V-potential-static holding; however, Pt/C degrades mainly through the dissolution/loss and dissolution/redeposition process if stressed under Pstep(1.4V_150s-0.60V30s). The difference in the degradation mechanisms is attributed to the chemical states of Pt nanoparticles: Pt dissolution can be alleviated by the protective oxide layer under the Pstep(1.4V_150s-0.85V_30s) condition and the potential-static holding conditions. These findings are very important for understanding PEM fuel cell electrode degradation and are also useful for developing fast test Protocol for screening durable catalyst Support materials. (c) 2008 Elsevier B.V. All rights reserved.