This work reports the application of an activated clay mineral as adsorbent for the removal of a basic dye, methylene blue (MB). from aqueous solutions. The thermal treatment at 300 degrees C for 2 h and the acid activation with nitric acid of 0.5 mol/dm(3) under reflux conditions improve the adsorption capacity of the raw clay mineral. A maximum of 500 mg/g of MB at equilibrium is achieved. Equilibrium data are mathematically modelled using the Freundlich, Langmuir and Toth isotherm adsorption models. (C) 2009 Elsevier B.V. All rights reserved.