Novel sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 2,2'-bis(4-aminophenoxy)biphenyl-5,5'-disulfonic acid (oBAPBDS) with nonlinear configuration, and common nonsulfonated diamines. Water uptake (WU) in liquid and vapor, water stability, and proton conductivity (T of the resulting SPI membranes were investigated. They were soluble in in-cresol and dimethylsulfoxide, and their WUs in liquid were much larger than those of the SPIs from other sulfonated diamines with linear configuration such as 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (BAPBDS). NTDA-oBAPBDS membrane was soluble in water at room temperature, whereas all the oBAPBDS-based copolyimide membranes were insoluble in water and maintained mechanical strength after being soaked in distilled water at 80 degreesC for 40-1000 h. This much improved water stability was due to the enhanced solubility stability of membrane toward water. The water vapor sorption isotherms were rather similar between the SPIs with the nonlinear and linear configurations of sulfonated diamine moieties. The present SPIs with IECs of 1.8-2.6 meq/g, including NTDA-BAPBDS, showed reasonably high proton conductivities under the highly humid conditions and roughly fell on the same sigma-WU relation line. (C) 2004 Wiley Periodicals, Inc.