Organophilic montmorillonite (MMT) was prepared by on exchange between Na+ ions in the clay and twin benzyldimethyloctadecylammonium bromine cations in an aqueous medium. The organophilic MMT particles were easily dispersed and swollen in styrene monomer. Polystyrene-MMT nanocomposites were prepared by the free-radical polymerization of styrene containing dispersed clav. The intercalation spacing in the nanocomposites and the degree of dispersion of these composites were investigated with X-ray diffraction and transmission electron microscopy, respectively. The nanocomposites had higher weight-average molecular weights, lower glass-transition temperatures, and better thermal stability (the decomposition temperature was improved by ca. 70° C) than the virgin polystyrene. The rheological behavior of the polystyrene-MMT nanocomposites was also studied. © 2005 Wiley Periodicals, Inc.