Na-montmorillonite was modified by cetyltrimethylammonium bromide (CTAB) to enlarge the distance between the two silicate layers and to improve these silicate layers' organophilicity. The organic intercalation reagent had intercalated into the galleries of montmorillonite as determined by FTIR and wide angle X-ray diffraction (WAXD). Then, polystyrene (PS)/clay nanocomposites (PSCN) were successfully prepared by means of in situ intercalative polymerization using supercritical carbon dioxide (SC CO2) as monomer/initiator carrier. During the process, both SC CO2/montmorillonite binary system and SC CO2/monomer/montmorillonite ternary system were studied. Exfoliated and intercalated PSCN can be obtained, respectively, by adjusting the content of clay in PS matrix, which was confirmed by WAXD and by transmission electron microscopy (TEM). Thermal analysis performed with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) indicates PSCN has superior thermal stability to pristine PS. (c) 2006 Wiley Periodicals, Inc.