Novel polybenzimidazole (PBI)/montmorillonite (MMT) nanocomposite membranes were prepared from an organosoluble, fluorine-containing PBI with an organically modified MMT (m-MMT) clay. Both wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) analyses showed that the m-MMT was well dispersed in the PBI matrix on a nanometer scale. The thermooxidative stability of PBI membranes increased slightly with the increase of m-MMT content. The coefficients of the thermal expansion (CTE) of PBI/7 wt% m-MMT nanocomposite membranes were decreased by 30% relative to that of pure PB1. The mechanical properties and the methanol barrier ability of the PBI films were significantly improved by the addition of m-MMT. The tensile modulus of PBI/5 wt% m-MMT nanocomposite membranes had a 41 % increase compared to the pure PBI films. The m-MMT in the phosphoric acid-doped PBI could effectively inhibit the plasticizing effect of the phosphoric acid. The methanol permeability of the PBI/5 wt% m-MMT nanocomposite membrane decreased by approximately 81 % with respect to the pure PBI membranes. The conductivity of the acid-doped PBI/m-MMT nanocomposites was slightly lower than the acid-doped pure PBL (C) 2007 Elsevier B.V. All rights reserved.