A series of side-chain-type sulfonated polyimides (SPIs) were synthesized from 4,4'-ketone dinaphthalene-1,1',8,8'-tetracarboxylic dianhydride (KDNTDA), 2,2'-(4-sulfophenoxy) benzidine, and nonsulfonated diamines. The SPIs showed anisotropic membrane swelling in water with 2-4 times larger swelling in thickness direction than that in plane one, being much different from SPIs derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride. They exhibited good solubility in dimethyl sulfoxide and 1-methyl-2-pyrrolidone even in proton form, which is favorable for processing in fuel cell applications. KDNTDA-based SPIs displayed good mechanical properties with reduced viscosities ranged from 1.1 to 2.4 dl g(-1) at 35 A degrees C in triethylamine salt form. They also showed reasonably high proton conductivity and thermal stability.