A novel sulfonated polybenzimidazole. sulfonated poly [2,2'-(p-oxydiphenylene)-5,5'-bibenzimidazole] (SOPBI), was successfully prepared by post-sulfonation reaction of the parent polymer, poly [2,2'-(p-oxydiphenylene)-5,5'-bibenzimidazole] (OPBI), using concentrated and fuming sulfuric acid as the sulfonating reagent at 80 degrees C, and the degree of sulfonation (DS) could be regulated by controlling the reaction conditions. No significant polymer degradation was observed in the post-sulfonation processes. Direct polymerization of 4,4'-dicarboxydiphenyl ether-2,2'disulfonic acid disodium salt (DCDPEDS) and 3,3'-diaminobenzidine (DABz), however, resulted in insoluble gels either in polyphosphoric acid (PPA) or in phosphorus pentoxide/methanesulfonic acid (PPMA) in a ratio of 1: 10 by weight reaction medium. The SOPBIs prepared by the post-sulfonation method showed good solubility in dimethyl sulfoxide (DMSO), high thermal stability, good film forming ability and excellent mechanical properties. Cross-linked SOPBI membranes were successfully prepared by thermal treatment of phosphoric acid-doped SOPBI membranes at 180 degrees C in vacuo for 20 h and the resulting cross-linked membranes showed much improved water stability and radical oxidative stability in comparison with the corresponding uncross-linked ones, while the proton conductivity did not change largely. Highly proton conductive (150 mS cm(-1). 120 degrees C in water) and water stable SOPBI membrane was developed. (C) 2007 Elsevier Ltd. All rights reserved.