The adsorption characteristics of methylene blue (MB) onto spent activated clay (SAC), a waste produced from an edible oil manufacturer was investigated. Results showed that the adsorption increased with increasing MB concentration, temperature, and pH. The adsorption equilibrium data was well fitted by multilayer adsorption isotherm. The maximum adsorption capacities for MB ranged from 0.94 x 10(-4) to 3.41 x 10(-4) mol/g between 5 and 45 degrees C. Thermodynamic parameters suggest that the adsorption is spontaneous and endothermic. We proposed a modified double exponential equation accounting both with chemical and mathematical point of view to describe the adsorption kinetic data. The increases of mass transfer and adsorption capacity were mainly. attributed to the interlayer of the SAC expanding at higher temperature. An activation energy of 13.5 kcal/K mol was determined suggesting that the adsorption involved a chemical reaction mechanism. (C) 2006 Elsevier B.V. All rights reserved.