The scope of this study was ammonium ions removal from synthetic aqueous solutions by raw and pretreated natural zeolite, Transcarpathian mordenite under static and dynamic conditions. The cation exchange capacity of the Transcarpathian mordenite regarding ammonium ions was evaluated as 1.64 meq/g at 1000 mg/l initial NH4-N concentration. The dynamic exchange capacity exceeded one estimated in equilibrium study at the same initial concentration that may be conditioned by the constant removal of ion exchange products. Ammonium C. Efficiency of uptake rate was controlled by particle diffusion with diffusion coefficients determined in the range of 0.7-3.6 x 10(-12) m(2)/s ammonium sorption may be improved by slowing down of initial solution rate in the column test and non-significantly by NaCl and HCl pretreatment of the mordenite. Ammonium sorption by the mordenite increased from the coarser to the finer fraction but this dependence became weaker to low flow rates. It was established that hydrogen ions displaced exchangeable cations on the mordenite in distilled water and hydrochloric acid with destroy of the zeolite framework structure in the last case. NH4+-ions removal from aqueous solutions occurs mainly by ion exchange with Na+ and Ca2+-ions at the practically equal parts of them because of the weakest affinity of the mordenite to these cations. (C) 2005 Elsevier B.V. All rights reserved.