The local natural clays from Valea Chiuoarului (kaolinite, illite, montmorillonite, nontronite, with small amounts of quartz) and Rãzoare areas (montmorillonite, kaolinite, illite, feldspar, clinoptilolite and quartz) were tested for the ability to adsorb ammonium ions from aqueous solutions. To increase the adsorption and cation-exchange capacity (CEC), the clays were treated with sodium chloride. The effect of initial concentration of ammonium ions on removal efficiency was studied. The equilibrium data were fitted with the Langmuir and Freundlich isotherm models. The Langmuir model described the experimental data better than the Freundlich model. The highest ion exchange capacity toward ammonium was displayed by the Na-exchanged clays.