The adsorption of the cationic surfactant BDDAC on a hydrophilic smectite (montmorillonite) surface has 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 surfactant coions (Cl(-)) were found to be adsorbed simultaneously with the cationic part after a complete alkyl ammonium ion-exchange of montmorillonite (CEC). We observe that organoclay particles remain flocculated in aqueous medium in almost all the range of adsorption isotherms up to 1.38 CEC and afterward redisperse incompletely. The intercalation of surfactant in the interlamellar space was followed by X-ray measurements. Fluorescence spectroscopy was used to obtain information about the adsorption layer at the interface. A calculation from geometrical considerations and from adsorption isotherms shows that below 1.38 CEC, there is a flat double layer in the interlamellar space and after this amount of adsorbed surfactant, an interstratified clay-surfactant mixed system is formed. Copyright 1999 Academic Press.