Isotherms of phenanthrene adsorption on different organoclay complexes were obtained using the HPLC technique to understand the adsorption behavior and to characterize the effect of sodium chloride (NaCl) on the adsorption. The adsorbed amounts of phenanthrene on montmorillonite exchanged by organic cations such as tetraheptylammonium, benzyltrimethylammonium, hexadecyltrimethylammonium, or tetraphenylphosphonium were several times higher than those obtained using montmorillonite clay without surface modification. At the same equilibrium concentration, the adsorbed amount of phenanthrene is higher on clay modified with benzyltrimethylammonium than on clay modified with hexadecyltrimethylammonium or other cations. Adsorption of phenanthrene on clay modified with benzyltrimethylammonium increased dramatically as the concentration of NaCl increased up to 150 g/l in the aqueous solution. The shape of the curves obtained can be classified as S-type. The adsorption data obtained from salinity experiments support a mathematical model that link:; the Langmuir constant with the salinity constant. This model may be useful to predict the equilibrium concentration of a contaminant in saline solution. FTIR studies showed strong interactions between the aromatic rings of phenanthrene and the preadsorbed benzyltrimethylammonium on clay surfaces. (C) 2003 Elsevier Inc. All rights reserved.