Montmorillonite/polypyrrole (MMT/PPy) nanocomposites were prepared by the in situ polymerization of pyrrole in the presence of MMT. The morphology of the MMT/PPy nanocomposites as examined by scanning electron microscopy differs slightly from that of the untreated MMT but markedly from that of polypyrrole. X-ray photoelectron spectroscopy (XPS) showed that the materials have MMT-rich surfaces, an indication that polypyrrole is essentially intercalated in the host clay galleries. The transmission electron microscopy showed, that the interlamellar spacing of the untreated MMT increased from 1.25 to 18.9 nm, when compared to nanocomposite MMT/10.8% PPy. Moreover, XPS highlighted the cation exchange of Na+ from montmorillonite by K+ (from the oxidant) and by the positively charged polypyrrole chains. Inverse gas chromatography indicated that the nanocomposites are high surface energy materials with a dispersive contribution to the surface energy (gamma(s)(d)) reaching 200 mJ/m(2) at 150 degrees C, for a PPy loading of 21.4 wt%. The (gamma(s)(d)) values of the MMT/PPy nanocomposites were correlated to the changes in the specific surface area of the MMT induced by the intercalation of polypyrrole. (c) 2005 Elsevier Ltd. All rights reserved.