Organophilic montmorillonite was prepared using ion-exchange method between sodium ions in clay layers and stearyltrimethyl ammonium chloride in the various solvents, including deionized water, ethanol, acetone, and toluene. The montmorillonite has the largest d(001) spacing, as determined by X-ray diffraction in toluene, than the other solvents considered. Ethanol can completely wash out the overexchanged stearyltrimethyl ammonium chloride among layers of montmorillonite. However, deionized water is the preferred ion-exchange solvent. The thermal stability of organophilic montmorillonite was investigated by high-resolution thermogravimetric analysis (TGA). Polystyrene-montmorillonite nanocomposites were obtained by suspension free radical polymerization of styrene in the dispersed organophilic montmorillonite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that montmorillonite had been exfoliated. 5.0 wt % of clay in the synthesized nanocomposite was found to be the optimum content that improved both thermal and mechanical properties over those of pure polystyrene under the experimental conditions applied. (C) 2003 Wiley Periodicals, Inc.