A series of sulfonated polyimides with increasing alkyl substituents in the o-position to diamine were synthesized from 4,4'-methylene dianiline, 4,4'-diamine-3,3'-dimethyl-diphenylmethane, and 4,4'-diamine-3,5,3',5'-tetraethyl-diphenylmethane using 1,41,5,8-naphthalenetetracarboxylic dianhydride and perylenetetracarboxylic dianhydride by chemical imidization method. 4,4'-Diaminobiphenyl 2,2'-disulfonic acid was used as; sulfonated diamine. The variation in the membrane properties with increase in substitution was analyzed. Solubility increased with substitution whereas the thermal stability decreased with increase in substitution. Ion exchange capacity and water uptake reduced with increase in substitution because of the low sulfonic acid content at a particular weight due to the increased molecular weight of the repeating unit. The conductivity of the substituted diamines was higher than the unsubstituted diamines at higher temperature regardless of low ion exchange capacity and water uptake. The increase in conductivity with increase in temperature was more rapid in polyimides than in Nafion((R))115. Hydrolytic stability of the polyimides with substitution is more than the unsubstituted diamines. (C) 2004 Wiley Periodicals, Inc.