A novel sulfonated diamine monomer, 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (BAPBDS) with the high basisity and flexible structure was synthesized by direct sulfonation of 4,4'-bis(4-aminophenoxy)biphenyl (BAPB). Sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPBDS and nonsulfonated diamines such as BAPB. The SPI membranes showed much higher water stability at high temperatures than other sulfonated diamine-based SPIs reported so far. Their water vapor sorption isotherm, water uptake (WU), density, dimensional change and proton conductivity sigma were investigated. The SPIs showed rather isotropic dimensional changes with WU and the volume increases were slightly smaller than those estimated from the additivity. The SPIs with ion exchange capacities (IECs) of 1.9-2.7 meq/g displayed the similar relationship between sigma and WU each other, which was different from those of Nation H 7 and also of the SPIs with the lower IECs. The former SPIs showed reasonably high sigma values of 10(-2) S/cm or more even at WU of 25 g/100 g dry polymer under 70% RH at 50degreesC, whereas the latter showed the similarly high sigma values only in liquid water, but not in the nearly saturated water vapor. (C) 2003 Elsevier B.V. All rights reserved.