Polyaniline (PANI)-montmorillonite (MMT) nanocomposites were prepared by direct intercalation of aniline molecules into MMT galleries, followed by in situ polymerization within the nano-interlamellar spaces under solvent-free conditions. The basal spacing of aniline-intercalated MMT increased gradually up to 1.5 nm with increasing amounts of aniline loaded. This result suggests that aniline molecules were adsorbed by MMT clay and that intercalated aniline likely located perpendicular to the silicate sheets. After polymerization, X-ray diffraction and Fourier transform infrared analyses confirmed the successful synthesis of PANI chains between the MMT nano-interlayers. The scanning electron microscopy images indicated that the surface morphologies of PANI-MMTs were strongly different depending on the PANI content. The electrical conductivities of PANI nanocomposite particles in pressed pellets ranged in the order of between 10(-3) and 10(-2) S/cm. UV-vis spectroscopy and doping level measurement were further used to discuss the conductivities of nanocomposites. The thermal stabilities of PANI-MMT nanocomposites were examined by using thermogravimetric-differential thermal analysis and derivative thermogravimetric analysis, and both analyses consequently demonstrated the improved thermal stabilities of the PANI chains in the nanocomposites as compared to pure PANI. The thermal stabilities of resulting nanocomposites were strongly related to the PANI content, which increased as the PANI content decreased in the nanocomposites. (c) 2005 Wiley Periodicals, Inc.