A series of high molecular weight, sulfonated polyimide copolymers (8a-f) with controlled acid contents have been obtained using 2,2 '-bis(4-sulfobenzyloxy)benzidine (14) prepared via a flexible synthetic route. This series of novel sulfonated polyimide membranes were found to possess higher hydrolytic stability than polyimides in which the sulfonic acid groups are bound directly to the polymer main chain. An in-depth analysis of conductivity data was also performed for 8 and compared to the results for Nafion (R) (1), sulfonated poly(ether ether ketone) (2) and a main-chain sulfonated polyimide (3). In order to remove the influence of acid strength, the proton mobility value for 8 at infinite dilution was calculated and found to be 1.2(+/- 0.6) x 10(-3) cm(2) s(-1) V-1. A catalyst-coated membrane (CCM)-MEA based on a polyimide incorporating 60% sulfonated monomer (8d) was found to exhibit comparable beginning-of-life fuel cell performance as a Nafion (R)-based CCM MEA at 50 degrees C. (C) 2008 Elsevier Ltd. All rights reserved.