Studies in fuel cell membranes show that modification of polyimides by introduction of aliphatic linkages in the structure of sulfonated copolyimides, synthesis of branched/crosslinked sulfonated polyimides, and semi and fully interpenetrating polymer networks of sulfonated polyimides restrain suitable potential for Nafion substitution. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.