Phosphoric acid doped polybenzimidazole (PBI) and PBI composite membranes have been prepared in the present work. The PBI composites contain inorganic proton conductors including zirconium phosphate (ZrP), (Zr(HPO4)(2).nH(2)O), phosphotungstic acid (PWA), (H3PW12O40.nH(2)O) and silicotungstic acid (SiWA), (H4SiW12O40.nH(2)O). The conductivity of phosphoric acid doped PBI and PBI composite membranes was found to be dependent on the acid doping level, relative humidity (RH) and temperature. A conductivity of 6.8 x 10(-2) S cm(-1) was observed for PBI membranes with a H3PO4 doping level of 5.6 (mole number of H3PO4 per repeat unit of PBI) at 200 degreesC and 5% RH. A higher conductivity of 9.6 x 10(-2) S cm(-1) was obtained by composite of 15 wt.% of ZrP in a PBI membrane under the same conditions. Homogeneous membranes with good mechanical strength were prepared by introducing PWA (20-30 wt.%) and SiWA (20-30 wt.%) into PBI, and their conductivity were found to be higher than or comparable with that of the PBI membrane at temperatures up to 110 degreesC. (C) 2003 Elsevier B.V. All rights reserved.