A series of sulfopropylated and sulfobutylated polyimide copolymers containing fluorenyl groups, SPI-4, were synthesized to investigate the effect of alkyl side chains on the properties (stability, mechanical strength, water uptake, and proton conductivity) of the polymimide electrolyte membranes. SPI-4 showed much better hydrolytic stability (in 10% MeOH aq at 100 degrees C) than the main chain sulfonated polyimide, SPI-1. Tough, flexible, and ductile membranes were obtained from these copolymers. At high relative humidity all the SPI-4 membranes showed high mechanical properties (> 34 MPa of the maximum stress) and proton conductivity (> 0.1 Scm(-1)). These properties are comparable to or even better than those of the perfluorosulfonic acid ionomer (Nafion 112). The new polyimide ionomers have proved to be a possible candidate as polymer electrolyte membrane for PEFCs and DMFCs. (c) 2005 Wiley Periodicals, Inc.