A novel sulfonated diamine bearing sulfophenyl pendant groups of 2,2 '-(4-sulfophenyl) benzidine (BSPhB) was synthesized. Sulfonated polyimides (SPIs) derived from 1,4,5,8-naphthalene tetracarboxylic dianhydride, BSPhB and other non-sulfonated diamines were successfully synthesized. The SPIs with ion exchange capacity (IEC) of 1.5-2.8 meq g(-1) showed high reduced viscosity of 3-10 dL g(-1) and high desulfortation temperature of 320 degrees C. The SPI membranes were tough and flexible, having high stress at break of more than 80 MPa and elongation of 80-100%. They showed highly anisotropic membrane swelling in water with larger swelling in thickness direction than in plane direction. They showed fairly high proton conductivity (sigma). For example, the membrane with IEC of 1.77 meq g(-1) exhibited a values of 120 and 260 mS cm(-1) 60 and 120 degrees C, respectively, in water. They also showed fairly high water stability. (c) 2007 Elsevier Ltd. All rights reserved.