Good dispersion of modified laponite clay was achieved in polybenzimidazole (PBI) solutions which, when cast and allowed to dry, resulted in homogeneous and transparent composite membranes containing up to 20 wt% clay in the polymer. The clay was organically modified using a series of ammonium and pyridinium salts with varying polarity and hydrogen-bonding capacity. Clay modification by ion-exchange reactions involving replacement of interlayer inorganic cations was confirmed using X-ray photoelectron and infrared spectroscopy techniques. The cast PBI membranes were characterized by their water uptake, acid doping and swelling, tensile strength, conductivity and hydrogen permeability as well as by fuel cell tests. For the composite membranes, high acid doping levels were achieved with sufficient mechanical strength and improved dimensional stability or reduced membrane swelling. At an acid doping level of 12 mol H3PO4 per monomer unit, proton conductivity as high as 0.12 S cm(-1) was obtained at 150 degrees C and 12% relative humidity. The composite membranes exhibited hydrogen permeability ranging from 0.6 to 1.2 x 10(-10) mol cm(-1) s(-1) bar(-1) from 100 to 200 degrees C, which was five times lower than that of acid-doped pristine PBI membranes. In accordance with the hydrogen permeability measurements, fuel cell tests exhibited high open circuit voltages (i.e., 1.02 V) at room temperature as well as high I-V performance compared with normal PM membranes. (C) 2011 Elsevier B.V. All rights reserved.