Nanocomposites of poly(methyl methacrylate) (PMMA) with layered silicates were prepared by the melt mixing route. The PMMA-montmorillonite hybrids were characterized by wide-angle X-ray diffraction, thermo-gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Except for the hybrid formed with unmodified sodium montmorillonite, where a change in distribution of clay platelet stacks was observed without polymer intercalation, the use of organically modified montmorillonites produced well-intercalated systems. An increase in the separation of the clay layers due to polymer intercalation was observed in the range 7-14 Angstrom. TGA thermographs indicated that the onset of decomposition increased by 15-30degreesC, depending on the organoclay, as compared to PMMA itself. DSC results showed the existence of a glass-transition temperature (T-g) for all the hybrids and also showed that the T-g of the hybrids were lower (by similar to10%) compared to that of PMMA. Among the various modifiers, the ones that were relatively more polar favored a greater extent of intercalation of PMMA into the clay layers. The thermal stabilities of the nanocomposites were superior to that of PMMA and were controlled by the stability of the organoclay and its interaction with PMMA. (C) 2003 Wiley Periodicals, Inc.