A series of novel sulfonated polyimides (equivalent weight per sulfonic acid = 310-744 g/equiv) containing 10-70 mol % 1,5-naphthylene moieties were synthesized as potential electrolyte materials for high-temperature polymer electrolyte fuel cells. The polycondensation of 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4'-diamino-2,2'-biphenyldisulfonic acid, and 1,5-diaminonaphthalene gave the title polymer electrolytes. The polyimide electrolytes were high-molecular-weight (number-average molecular weight = 36.0-350.7 x 10(3) and weight-average molecular weight = 70.4-598.5 x 10(3)) and formed flexible and tough films. The thermal properties (decomposition temperature > 260 degreesC, no glass-transition temperature), stability to oxidation, and water absorption were analyzed and compared with those of perfluorosulfonic acid polymers. The polyimide containing 20 mol % 1,5-naphthylene moieties showed higher proton conductivity (0.3 S cm(-1)) at 120 degreesC and 100% relative humidity than perfluorosulfonic acid polymers. The temperature and humidity dependence of the proton conductivity was examined. (C) 2003 Wiley Periodicals, Inc.