In this study, the ability of a Croatian clinoptilolite- and montmorillonite-rich tuffs to take up ammonium from aqueous solutions was investigated. Montmorillonite tuff was converted into the Na + and H + form by treatment with Na 2 CO 3 (w = 3-5%) and the solution of H 2 SO 4 (w = 15%). The properties of clay modified in this way were investigated as well. The results show that the clinoptilolite tuff efficiency for ammonium removal is far superior (by 45-50%) to the removal efficiency of montmorillonite tuff. Treatment of montmorillonite tuff with Na 2 CO 3 resulted in a slight improvement of its ammonium exchange efficiency. Acid modification of montmorillonite tuff causes a considerable decrease of ammonium exchange. Maximum loadings in ammonium exchange in clinoptilolite tuff, montmorillonite tuff, alkaline modified montmorillonite tuff, and acid modified montmorillonite tuff, as calculated by the Langmuir equation, are 1.20, 0.54, 0.64, and 0.21 mmol g - 1 NH 4 + , respectively.