Semi-interpenetrating polymer network (semi-IPN) membranes based on novel sulfonated polyimide (SPI) and poly (ethylene glycol) diacrylate (PEGDA) have been prepared for the fuel cell applications. SPI was synthesized from 1,4,5,8-napththalenetetracarboxylic dianhydride, 4,4'-diaminobiphenyl 2,2'-disulfonic acid, and 2-bis [4-(4-aminophenoxy) phenyl] hexafluoropropane. PEGDA was polymerized in the presence of SPI to synthesize semi-IPN membranes of different ionic contents. These membranes were characterized by determining, ion exchange capacity, water uptake, water stability, proton conductivity, and thermal stability. The proton conductivity of the membranes increased with increasing PEGDA content in the order of 10(-1) S cm(-1) at 90 degrees C. These interpenetrating network membranes showed higher water stability than the pure acid polyimide membrane. This study shows that semi-IPN SPI membranes based on PEGDA which gives hydrophilic group and structural stability can be available candidates comparable to Nafion (R) 117 over 70 degrees C. (c) 2007 Wiley Periodicals, Inc.