A simple model employing a linear driving force in the particle phase has been successfully employed to model ammonium removal from a synthetic wastewater by a column of clinoptilolite. Batch isotherm studies conducted with the ammonium containing wastewater where shown to follow the form of a Langmuir isotherm for a single solute system but these data predicted better column performance than that observed. This was shown to be caused by incomplete regeneration of the clinoptilolite column. However, the column performance could be modeled effectively using an equilibrium isotherm constructed from column capacity data collected with different influent ammonium concentrations. The column isotherm was only valid for the particular conditions of regeneration employed in this study. Subsequent studies on more completely regenerated clinoptilolite indicate that the initial distribution of ammonium on the clinoptilolite must be identified if the column performance is to be modeled more accurately.