Adsorption isotherms for an ammonium ion-sepiolite system have been established for initial ammonium ion solution concentrations in the range of 8.32-388.06 mmol NH4+ /L (150-7000 ppm). The change of the "g sepiolite/mL solution" ratios from 1/10 to 1/70 caused an increase in the adsorption from 1.82 to 3.70 mmol NH4+ /g for the average particle size of 2.8 mm. The equilibrium data were fitted to "general-purpose isotherms" and "first-order adsorption model", and the adjustable parameters of the isotherms were estimated by nonlinear least-squares analysis. Langmuir and Freundlich models were found insufficient to explain the adsorption equilibrium, while Langmuir Freundlich and Tooth isotherms explained the data well. Goodness of fit increased with the increased sepiolite quantity in the treated suspension. (C) 2004 Elsevier Ltd. All rights reserved.