Polyolefins and chlorine-containing polymers were investigated to produce polymer nanocomposites. Natural and organic-treated montmorillonite clays were melt compounded with the polymers. Organic-treated montmorillonite clay dispersed well in polychloroprene, chlorinated-polyethylene, polyvinyl chloride, chlorinated-polyvinyl chloride, and polyvinylidenechloride polymers, and formed nanocomposites. They were not well dispersed in polyolefins that contain no chlorine. X-ray diffraction and transmission electron microscopy techniques indicated the separation of moritmorillonite layers and indicated the formation of polymer nanocomposites in chlorine-containing polymers. Mechanical testing showed enhanced tensile strength and Young's modulus of chlorinated-polymers/clay compounds, but not polyolefins/clay compounds. (C) 2003 Wiley Periodicals, Inc.