Montmorillonite, organically modified by octadecylammine salt, has been adopted to successfully fabricate the exfoliated organic montmorillonite/poly(3,4-ethyldioxythiophene) (OMMT/PEDOT) nanocomposites by in situ polymerization in aqueous media. Hydrochloric acid, 1,5-naphthalenedisulfonic acid, and sodium benzenesulphonate have been employed to activate the polymerization of 3,4-ethyldioxythiophene by offering active sites on the layers of montmorillonite. The resulting exfoliated nanocomposites have been characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, and electrical conductivity measurement and showed controllable conductivity in the range of 10(-7) to 10(-2) S/cm and improved thermal stability compared with pure PEDOT. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2400-2407, 2009