A novel sulfonated polyimide membrane containing triazole groups (SPI-8) was subjected to long-term fuel cell operation. Excellent durability of the SPI-8 membrane was confirmed by single cell operation for 5000 h at 80 degrees C. Open circuit voltage and hydrogen crossover through the membrane showed only minor changes during cell operation, indicating a lack of catastrophic damage for the SPI-8 membrane. It was found by post-test analyses of the membrane that the ion exchange capacity (IEC) decreased only slightly, but the molecular weight decreased to 1/10, resulting in a loss of mechanical strength. It was concluded that the major degradation mode of the sulfonated polyimide membrane involves the ring-opening of the imide linkages via hydrolysis, while a certain degree of side chain degradation occurs as a result of oxidative attack by radical species. (c) 2008 Elsevier Ltd. All rights reserved.