In this paper, a 600 h life test of a high temperature PEMFC based on phosphoric acid (H3PO4)-doped polybenzimidazole (PBI) (H3PO4/PBI HT-PEMFC) at a current density of 714 mA cm(-2) (the beginning 510h continuous test) and 300 mA cm(-2) (the last 90 h intermittent test) was carried out. After the life test, degradation of the MEA occurred. The H-2 crossover rate through the PBI membrane and the open circuit voltage (OCV) of the cell were tested with time. The results showed that, at the beginning of 510 h continuous test, the PBI membrane did not show much physical degradation, but during the last 90 h test there was a remarkable physical degradation which resulted in a higher H-2 crossover. The catalysts, PBI membranes and the membrane electrode assemblies (MEAs) before and after the life test were comprehensively examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). TEM results showed that the particle size of the Pt/C catalysts in the anode and cathode increased from 3.72 to 7.40 and 8.39 nm, respectively. SEM images of MEA in cross-section revealed that the PBI membrane became thin after the life test. EDS analysis implied the leaching of H3PO4 from the PBI membrane had occurred. Therefore, we conclude that physical degradation of PBI membrane, agglomeration of the electrocatalysts (both anode and cathode) and the leaching of H3PO4 from the PBI membrane were responsible for the performance degradation of the H3PO4/PBI HT-PEMFC. (c) 2006 Elsevier B.V. All rights reserved.