A series of sulfonated copolyimides (co-SPIs) bearing pendant sulfonic acid groups were synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), bis(3-sulfopropoxy) benzidines (BSPBs), and common nonsulfonated diamines via statistical or sequenced polycondensation reactions. Membranes were prepared by casting their m-cresol solutions. The co-SPI membrane had a microphase-separated structure composed of hydrophilic and hydrophobic domains, but the connecting behavior of hydrophilic domains was different from that of the homo-SPIs. The co-SPI membranes displayed clear anisotropic membrane swelling in water with negligibly small dimensional changes in the plane direction of the membrane. With water uptake values of 39-94 wt %, they showed dimensional changes in membrane thickness of about 0.11-0.58, which were much lower than those of homo-SPIs. The proton conductivity sigma, values of co-SPI membranes with ion exchange capacity values ranging from 1.95-2.32 meq/g increased sigmoidally with increasing relative humidity. They displayed sigma values of 0.05-0.16 S/cm at 50 degrees C in liquid water. Increasing temperature up to 120 degrees C resulted in further increase in proton conductivity. The co-SPI membranes showed relatively good conductivity stability during the aging treatment in water at 100 degrees C for 300 h. (c) 2005 Wiley Periodicals, Inc.