Polyurethane/clay nanocomposites were fabricated in solution and tested as gas barrier membranes. Natural montmorillonite was modified with various alkylammonium surfactants and incorporated into a polyurethane matrix. Permeation properties of the nanocomposites were studied as a function of processing methodology. Pre-processing clay samples with sonication instead of plain stirring led to significantly better barrier properties, suggesting that dispersion of the clay particles is a critical factor. Fitting the experimental permeabilities with phenomenological models, which predict the effective permeability of polymer systems filled with barrier flakes as a function of flake concentration, led to unrealistically low values of calculated aspect ratios of the clay layers. (C) 2009 Elsevier B.V. All rights reserved.