This paper presents the results on the Grand-Canonical Monte Carlo simulations of the adsorption of the paracresol uremic toxin and water into the silicalite-1 and pure silica faujasite zeolites. The co-adsorption of water and paracresol seems to proceed along a cooperation effect between the toxin and the solvent. A model of adsorption that accounts for the effect of the solvent has been elaborated and verified using experimental isotherms. The model is based on the Langmuir isotherm in which an apparent adsorption enthalpy is used that changes with the concentration of the solute. The new expression for the isotherm reproduces the experimental isotherm with good accuracy and physical interpretation is given to justify the model. (C) 2010 Elsevier B. V. All rights reserved.