The paper presents the results of investigating the treatment of leakage waters from waste dumps using activated carbon and natural zeolite clinoptilolite, known as a very selective and efficient cation exchanger for ammonium ions. The results are presented of chemical and physical analyses of leakage waters characterized by a high content of ammonium (820 mg L-1) and organic pollutants (1033 mg L-1 C). Physical and chemical characteristics of zeolite and the exchange of ammonium ions in model and real solutions were determined in laboratory trials. Treatment of leakage water with 0.04-2.5% (w/w) activated carbon (Norit 0.8 Supra) led to a reduction of total organic carbon in leakage water from 1033 to 5 10 mg L-1. Pretreatment of leakage water with activated carbon did not improve the exchange of ammonium ions on zeolite. Without pretreatment of leakage water, the exchange of ammonium ions amounted to 4.2 mg NH4+/g zeolite. Addition of activated carbon, regardless of its mass, increased the exchange of ammonium ions to only 5.7 mg NH4+/g zeolite. In the model solution of an equal concentration of ammonium as the real solution, 17.70 mg NH4+/g zeolite was exchanged. Organic pollutants that were not eliminated by activated carbon (most probably components of natural origin) adsorbed to zeolite and prevented the exchange of NH4+ ions, which was also reduced due to the presence of K+ and Ca2+ ions. (C) 2004 Elsevier B.V. All rights reserved.