Novel sulfonated diamines bearing aromatic pendant groups, namely, 3,5-diamino-3'-sulfo-4'-(4-sulfophenoxy) benzophenone (DASSPB) and 3,5-diamino-3'-sulfo-4'-(2,4-disulfophenoxy) benzophenone (DASDSPB), were successfully synthesized. Novel side-chain-type sulfonated (co)polyimides (SPIs) were synthesized from these two diamines, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and nonsulfonated diamines such as 4,4'-bis(3-aminophenoxy) phenyl sulfone (BAPPS). Tough and transparent membranes of SPIs with ion exchange capacity of 1.5-2.9 meq g(-1) were prepared. They showed good solubility and high thermal stability up to 300 degrees C. They showed isotropic membrane swelling in water, which was different from the main-chain-type and sulfoalkoxy-based side-chain-type SPIs. The relative humidity (RH) and temperature dependence of proton conductivity were examined. At low RH, the novel SPI membranes showed much higher conductivity than the sulfoalkoxy-based SPIs. They showed comparable or even higher proton conductivity than Nafion 112 in water at 60 degrees C (> 0.10 S cm(-1)). The membrane of NTDA-DASDSPB/BAPPS (1/1)-s displayed reasonably high proton conductivities of 0.05 and 0.30 S cm(-1) at 50 and 100% RH, respectively, at 120 degrees C. (c) 2006 Wiley Periodicals, Inc.