Monomer styrene and initiator N,N'-azobis-(isobutyronitrile) were impregnated into montmorillonite (MMT) galleries using supercritical CO2 at 35degreesC and 12.0 MPa, after thermal polymerization of monomer at 65degreesC, resulting in MMT/polystyrene nanocomposites. The morphology and structure of the products were characterized by FTIR, powder X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis. The results indicate that MMT is dispersed in the composite with intercalated and exfoliated structures, enhancing the thermal stability of nanocomposites. Changing the soaking time and the content of MMT in the supercritical solution during the impregnating process can control the exfoliated extent of MMT. (C) 2004 Wiley Periodicals, Inc.