The Bronsted acid-base composite membrane was prepared by entrapping benzimidazole in sulfonated poly(phenylene oxide) by tuning the doping ratios. Their thermal stability, dynamic mechanical properties and proton conductivity were investigated under the conditions for intermediate temperature proton exchange membrane (PEM) fuel cell operation. In addition, investigation of activation energies of the SPPO-xBnIm at different relative humidity was also performed. TG-DTA curves reveal these SPPO-xBnIm composite materials had the high thermal stability. The proton conductivity of SPPO-xBnlm composite material increased with the temperature, and the highest proton conductivity of SPPO-xBnlm composite materials was found to be 8.93 x 10(-4) S/cm at 200 degrees C under 35% relative humidity (RH) with a "doping rate" where x = 2. The SPPO-2BnIm composite membrane show higher storage moduli and loss moduli than SPPO. Tests in a hydrogen-air laboratory cell demonstrate the applicability of SPPO-2BnIrn in PEMFCs at intermediate temperature under non-humidified conditions. (C) 2007 Elsevier Ltd. All rights reserved.