In the pursuit of the hydrolytically stable sulfonated polyimide (SPI) membranes with high proton conductivity for fuel cell applications, a series of novel SPI ionomers derived from benzophenone-4,4'-bis(4-thio-1,8-naphthalic anhydride) (BPBTNA) were conveniently synthesized. The accelerated water stability tests demonstrated that the resultant SPI membranes kept highly the original mechanical properties even after 24 h in water at 140 degrees C. The membranes exhibited a microphase-separated structure with high morphological stability, and well-collected hydrophilic domains that could work as proton transport channels. The proton conductivity of 1c with an IEC of 1.90 meq g(-1) was higher than that of Nafion at 100% relative humidity (RH). (c) 2011 Elsevier Ltd. All rights reserved.