Adsorption isotherms for an ammonium ion-sepiolite system have been established for initial ammonium ion solution concentrations in the range of 8.32-388.06mmol NH 4 + /L (150-7000ppm). 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 3.70mmol NH 4 + /g for the average particle size of 2.8mm. 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.