Nanocomposites of polypropylene (PP)/ethylene-propylene-diene rubber (EPDM) blend with montmorillonite-based organoclay were prepared in a solvent blending method. Solvent blending of PP and EPDM in a composition of 50:50 formed a two phase morphology in which EPDM appeared as dispersed phase with irregular shape. The size of dispersed phase reduced significantly to almost spherical domains by addition of the nanoclay. For better dispersion of nanoclay in the PP/EPDM blend, an antioxidant was used as compatibilizer. State of nanoclay dispersion was evaluated by X-ray diffraction (XRD), and also, by a novel method using permeability measurements data in a permeability model. The measured d-spacing data proved a good dispersion of nanoclay at low clay contents along with compatibilizer. The permeability model for flake-filled polymers was used to estimate the aspect ratio of nanoclay platelets in the blend nanocomposites. Oxygen and carbon dioxide barrier property of the PP/EPDM blend improved about two-fold by adding only 1.5 vol% organoclay. Differential scanning calorimetry measurements indicated a decrease in crystallinity up to 27% suggesting a reduction in spherulites growth. However, the melting temperature remained unchanged. The increase in barrier property of the blend, despite a decrease in crystallinity, indicated the dominant role of organoclay platelets in barrier improvement. According to the permeability model, very high barrier property could be obtained if the aspect ratio of the flakes or platelets of the organoclay could be significantly increased in the blend. Scanning electron micrographs of fracture surface of nanocomposite membranes broken at very low temperatures, exhibited a very ductile surface indicating a good compatibility of PP and EPDM rubber and also, a possible contribution of nanoparticles to deformation mechanisms, such as extensive shear yielding in the polymer blend. (c) 2006 Elsevier B.V. All rights reserved.