AbstractPolyolefins 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 montmorillonite 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. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1581–1588, 2003