The adsorption constant of aqueous solution of methylene blue (MB), (800 . 10(-6) mol . dm(3)) on activated carbon (AC) has been investigated by means of UV-Visible spectroscopy. The adsorption process is very fast and is physical in nature; however, it is complicated by the factors inherent in the structures of both the MB solution and AC, such as tendency of MB to form molecular aggregates in solution. In this work, the effects of various experimental parameters such as temperature and ionic strength have been investigated, using a batch adsorption technique to obtain information on treating effluents from the dye industry. Utilizing the van't Hoff relation, which describes the dependence of equilibrium constant on temperature, as a constraint, we determine the spectral responses of the free and adsorbed amounts, as well as the enthalpy of the adsorption equilibrium. (c) 2006 Elsevier Ltd. All rights reserved.