Polyamide 6-montmorillonites membranes have been prepared and studied for a large range of clay content (from 0 to 18 wt.%). The barrier properties of these systems have been determined for different diffusing molecules varying by their kinetic diameter and their interaction capacity. The relative permeability has been found to be independent on the diffusing molecule showing that a tortuosity effect is at the origin of the improved barrier properties. The crystalline morphology of the polyamide matrix has been shown to be only slightly dependent on the nanocomposite composition. Consequently, the permeation properties have been related to the clay content and dispersion. From a quantitative description of the montmorillonite particle dispersion, the ability of different geometrical models to describe the experimental relative permeability data is discussed. This modelling leads to the conclusion that it is necessary to consider the polydispersity of the impermeable filler shapes and to take into account the presence of surfactant located at the inorganic surface to appropriately model the transport properties of the nanocomposites in a large range of nanoclay contents. (c) 2007 Elsevier B.V. All rights reserved.