X-ray diffraction has been used to study the changes in the surface properties of montmorillonitic clay through the changes in the basal spacings of sodium-montmorillonite (NaMt), acid-activated montmorillonite (AMt), pillared-montmorillonite (AlMt) and surfactant-intercalated organoclays. The basal spacing value of the NaMt increased from 12.01 to 18.1 angstrom by pillaring with Keggin ions ((hydroxyaluminum polycation) and until 21 angstrom by intercalation of the cationic surfactant in the interlayer space of the clay. Confirmations of the intercalated cationic surfactant have been characterized using Fourier transform infrared spectroscopy (FTIR). Thermogravimetric analysis shows that the thermal decomposition of montmorillonites modified with the cationic surfactant hexadecyltrimethylammonium (HDTMA) takes place in four steps. The surface areas of organo-montmorillonites were found to be much lower than that of raw montmorillonite. Surface areas of pillared and acid-activated montmorillonite are very high. This was explained by the emergence of the micropores and mesopores in the structure of the sample resulting from treatment. Adsorption of the 2,4,5-trichlorophenol (2,4,5-TCP) onto samples was studied. The greatest value of adsorption capacity of samples is attributed to the organo-montmorillonite (MtC16). (C) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.