The stability of Pt/C electrocatalyst was investigated by a series of intermittent life tests (100, 300 and 520 h) of single proton exchange membrane fuel cell (PEMFC) with phosphoric acid doped PBI (H3PO4/PBI) membranes at high temperature. The electrochemical surface area (ESA) loss of cathode electrocatalyst was investigated by electrochemical techniques (cyclic voltammetry, CV). During the first about 300 h, the ESA decreased fast from 17.2 to 7.8 m(2) g(Pt)(-1) (about 55%), and had only a small decrease (about 5%) in the following 210 h. The tested cathode catalysts were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) to determine the degree of electrocatalyst sintering. The results showed that the Pt particle size in the cathode increased from 4.0 to 8.3 nm before and after 300 It but it had no obvious growth in the further life tests. The tested MEA was also analyzed by energy dispersive X-ray (EDX) to investigate the platinum deposition in H3PO4/PBI membrane. Platinum did not be found in the tested H3PO4/PBI membrane. Above results indicated that the agglomeration of Pt particle should be responsible for the ESA loss of Pt/C electrocatalyst. The further analyses on TEM data suggested that agglomeration of platinum particles occurred via coalescence mechanism on carbon at the nanometer scale. (c) 2006 Elsevier B.V. All rights reserved.