The preparation of polystyrene (PS)/montmorillonite (MMT) composites in supercritical carbon dioxide (SC-CO2) was studied. Lipophilic organically modified MMT can be produced through an ion-exchange reaction between native hydrophilic MMT and an intercalating agent (alkyl ammonium). PS/clay composites were prepared by free-radical precipitation polymerization of styrene containing dispersed clay. X-ray diffraction and transmission electron microscopy indicated that intercalation of MMT was achieved. PS/clay composites have a higher thermal decomposition temperature and lower glass-transition temperature than pure PS. The IR spectrum analysis showed that the solvent of SC-CO2 did not change the structures of the PS molecules, but there were some chemical interactions between the PS and the clay in the composites. (c) 2005 Wiley Periodicals, Inc.