Poly(methyl methacrylate) and polystyrene/clay nanocomposites have been prepared via pseudo-dispersion polymerizations in the presence of a poly (dimethylsiloxane) surfactant-modified clay (PDMS-clay) in supercritical carbon dioxide. The effects of the PDMS-clay concentration on polymer conversion, molecular weight, and morphology have been investigated. The insoluble dispersion of PDMS-clay is shown to be an effective stabilizer for both MMA and styrene polymerization in ScCO2. The nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). While XRD shows featureless patterns for both nanocomposites, the actual distributions of clay are found to be quite different between PMMA and PS nanocomposites, presumably due to the different interaction mechanisms between the polymers and clay. Consequently, the different states of clay in the two nanocomposites play an important role in the mechanical properties of the nanocomposites, and a to a lesser degree in the thermal properties. (c) 2005 Elsevier Ltd. All rights reserved.