A novel proton conducting polymer electrolyte membrane, sulfonated polyimide having aliphatic moieties (SPI-5), was subjected to long-term fuel cell operation. High durability of the SPI-5 membrane was confirmed by single-cell testings at 80 degrees C for 5000 h. Still, the SPI-5 membrane turned out to be fragile after the cell operation. Through the investigation of the changes in thickness, ion exchange capacity, molecular weight, and NMR spectra, it was concluded that the most probable degradation mode of the sulfonated polyimide membrane should be the hydrolysis of imide linkages, while oxidative degradation is also likely to be involved. (C) 2006 The Electrochemical Society.