Chlorosulfonated polyethylene (CSPE)/organomontmorillonite (OMMT) nanocomposites were prepared by a melt intercalation method. The microstructure of the nanocomposites was characterized by transmission electron microscopy and X-ray diffraction. The effects of the OMMT content on the mechanical properties and swelling behavior of the nanocomposites were investigated. The improvement in the thermal stability of the nanocomposites was determined by thermogravimetric analysis. Transmission electron microscopy and X-ray diffraction showed that CSPE was intercalated into OMMT. When the OMMT content was lower than 12 wt%, the nanocomposites showed excellent tensile properties, which was attributed to nanometer-scale dispersion. The introduction of a small amount of OMMT also improved the thermal stability and swelling behavior, which was attributed to the gas barrier action of the OMMT layers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 3365-3368, 2010