An experimental polypropylene (PP) nanocomposite, containing approximately 4 wt % of an organophilic montmorillonite clay, was prepared and characterized, and its properties were compared with those of talc-filled (20-40 wt %) compositions. Weight reduction, with maintained or even improved flexural and tensile moduli, especially at temperatures up to 70degreesC, was a major driving force behind this work. By a comparison with the analytical data from a nylon 6 (PA-6) nanocomposite, it was found that the PP nanocomposite contained well-dispersed, intercalated clay particles; however, X-ray diffraction, transmission electron microscopy, dynamic mechanical analysis, and permeability measurements confirmed that exfoliation of the clay in PP was largely absent. The increased glass-transition temperature (T-g) of a PA-6 nanocomposite, which possessed fully exfoliated particles, indicated the molecular character of the matrix-particle interaction, whereas the PP nanocomposite exhibited simple matrix-filler interactions with no increase in T-g The PP nanocomposite exhibited a weight reduction of approximately 12% in comparison with the 20% talc-filled PP, while maintaining comparable stiffness. Undoubtedly, considerable advantages may be available if a fully exfoliated PP nanocomposite is fabricated; however, with the materials available, a combination of talc, or alternative reinforcements, and nanocomposite filler particles may provide optimum performance. (C) 2003 Wiley Periodicals, Inc.