The adsorption behaviour of hexadecylpyridinium bromide (CPB) onto bentonite was studied as a function of temperature under optimized condition of shaking time, and concentration of the adsorbate. Thermodynamic parameters such as DeltaHdegrees, and DeltaSdegrees were calculated from the slope and intercept of the linear plot of log K-d against 1/T. Based on the calculation of the thermodynamics of adsorption, it is evident that the principal contribution to the DeltaGdegrees of negative value (-10 to -14 kJ (.) mol(-1)) is the large positive value of DeltaSdegrees (58-60 J (.) K-1 (.) mol(-1)), whereas DeltaHdegrees is positive and in the range of 4-8 kJ (.) mol(-1). Analysis of adsorption results obtained at temperatures (288, 297, 303, 307, 313) K showed that the adsorption pattern on bentonite followed the Langmuir isotherm. The Langmuir sorption maximum at different temperatures for CPB sorption are reported (1100 to 1534 mmol (.) kg(-1)). The adsorption of the cationic surfactant CPB on a hydrophilic smectite (bentonite) surface has also been investigated, especially in the range of low coverage ratios where surfactant ions are adsorbed through cation-exchange with the counterions of the clay. The intercalation of surfactant in the interlamellar space was followed by X-ray measurements. At low surfactant concentrations, there is a peak corresponding to d(001) of 1.32 nm, and after full coverage at about 1.5 CEC (cation-exchange capacity) value, two peaks are seen corresponding to interlamellar distances of both (2.0 and 3.8) nm. Fluorescence spectroscopy was used to obtain information about the adsorption layer of HDTMA at the interface. (C) 2004 Published by Elsevier Ltd.