Polybenzimidazole (PBI) membranes have been prepared with different molecular weights. The water and acid swelling, mechanical strength, gas permeability and proton conductivity were studied for the pristine and acid doped PBI membranes. When doped with 5 mol of phosphoric acid per mole repeat unit of the polymer, a level necessary to obtain high enough proton conductivity for fuel cell uses, the polymer membrane exhibits a volume swelling by 118%, resulting in separation of the polymer backbones. The separation in turn reduces the mechanical strength of the membrane especially at high temperatures. Another consequence is the increased H-2 and O-2 permeability through the membrane. In the temperature range from 120 to 180 degrees C, the hydrogen permeability was found to be 1.6-4.3 x 10(-17) and 1.2-4.0 x 10(-15) Mol CM cm(-2) s(-1) Pa-1 for pristine and acid doped PBI membranes, respectively, while for oxygen it was 5.0-10 x 10(-19) and 3.0-9.4 x 10(-16) mol cm cm(-2) s(-1) Pa-1, respectively. High molecular weights of the polymers improve the mechanical strength but have little influence on the proton conductivity of the membranes. (c) 2005 Elsevier B.V. All rights reserved.