This study was performed to evaluate the adsorptive properties of red clay. Natural red clay was found to adsorb considerable amounts of Cr(VI) and phenol after modification by hexadecyltrimethylammonium bromide (HDTMA). The adsorptive properties of the modified natural red clay (HDTMA-clay) were investigated by batch studies, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The experiments showed that phenol and Cr(VI) ions can be simultaneously adsorbed on HDTMA-clay. Adsorption of phenol and Cr(VI) onto HDTMA-clay was tested as a function of temperature, pH, ionic strength, as well as initial Cr(VI) and phenol concentrations in solutions. The adsorption equilibrium was reached after 6 h for phenol and after 5 h for Cr(VI). The kinetics of phenol and Cr(VI) ions onto the red clay were best described by the first order kinetic model. The optimal pH range corresponding to the maximum adsorption of Cr(VI) on red clay was 4-6.5. A model with distribution constants of the H2CrO4, Cr2O72-, HCrO4- and CrO42- species to the adsorbent phase was used for the first time in the evaluation of the adsorption behavior of Cr(VI). In the case of phenol adsorption, the most effective pH range was pH > 8. The maximal adsorption capacity at pH 5.5 was 0.086 mmol/g and 0.012 mmol/g for Cr(VI) and phenol, respectively. The results showed that there were at least two adsorption sites in the red clay structure. (C) 2011 Elsevier B.V. All rights reserved.