This paper presents kinetic and equilibrium data concerning ammonium ion uptake from aqueous solutions using Romanian volcanic tuff. The influence of contact time, pH, ammonium concentration, presence of other cations and anion species is discussed. Equilibrium isotherms adequately fit the Langmuir and Freundlich models. The results showed a contact time of 3 h to be sufficient to reach equilibrium and pH of 7 to be the optimum value. Adsorption capacities of 19 mg NH4+/g were obtained in multicomponent solutions (containing NH4+, Zn2+, Cd2+ , Ca2+, Na2+). The presence of Zn and Cd at low concentrations did not decrease the ammonium adsorption capacity. Comparison of Romanian volcanic tuff with synthetic zeolites used for ammonium removal (5A, 13X and ZSM-5) was carried out. The removal efficiciency of ammonium by volcanic tuff were similar to those of zeolites 5A and 13X at low initial ammonium concentration, and much higher than those of zeolite ZSM-5. (c) 2006 Elsevier B.V. All rights reserved.