Removal of ammonium ion (NH4+) from leachate from Odayeri sanitary landfill, located in the European part of Istanbul, was examined using Gordes (Turkish) clinoptilolite. The effects of pH, flow rate, ammonium concentration and competitive cations on the removal efficiency were investigated in both batch and column studies. Thomas model were used to analyze the breakthrough data. Nonlinear least square method was applied for fitting the column data. The competitive effect was observed more effective in lower ammonium concentration and higher cation concentrations. Conditioning and regeneration using NaCl improved removal efficiency of ammonium from leachate. (c) 2007 Elsevier B.V. All rights reserved.