In this work, it is intended to study the effect of adsorbent concentration on the adsorption of phenol by hexadecyl trimethyl ammonium-bentonite. The experiments were conducted in two groups. The adsorption of hexadecyl trimethyl ammonium bromide (HDTMA) on bentonite was studied in the first group of experiments. It was observed that the all HDTMA was adsorbed by the bentonite, even when the amount used exceeded 100 % of cation exchange capacity (CEC). After the modification of bentonite by using HDTMA in an amount equivalent to 100 % of CEC, the adsorption experiments were performed at five different adsorbent concentrations ranging from 2 to 10 g/L. A type V isotherm and a non-linear increase in percent removal with adsorbent concentration were observed. The observation of the non-linear relation between the percent removal and adsorbent concentration was attributed to the effect of intra particle interactions and it was represented by a second order polynomial. Several adsorption isotherm equations were applied to the experimental data. Although, the Freundlich equation fitted fairly well, it failed to represent the plateau and the second region that appeared in the isotherm. Therefore, an equation giving the equilibrium concentration as a function of initial and adsorbent concentrations was suggested. © 2005 Elsevier B.V. All rights reserved.