A series of sulfonated polyimides (SPI) ionomers were synthesized from a new flexible dianhydride of 4,4'-ketone dinaphthalene tetracarboxylic dianhydride (KDNTDA), a sulfonated diamine of 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid and nonsulfonated diamines including dimethylsiloxane oligomer-based diamine. KDNTDA-based SPIs displayed good solubility in common aprotic solvents, which would provide an advantage of easy processing, being different from SPIs derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). They showed reasonably high proton conductivities and anisotropic membrane swelling with 2-3 times larger swelling in thickness than in plane. KDNTDA-based SPIs displayed slightly lower water stability than that of NTDA-based ones. On the other hand, the branched/crosslinked KDNTDA-based SPI membrane displayed higher water stability and kept high mechanical property and proton conductivity even after the aging in water at 130 degrees C for 300 h. (C) 2008 Elsevier B.V. All rights reserved.