Possibility of using natural zeolites for water softening was investigated. Quantitative data regarding separation of calcium from water at various levels of hardness through ion exchange with the ammonium selective natural zeolite clinoptilolite is reported. Capacity of the zeolite towards calcium removal in the presence of ammonium at low concentrations and calcium at higher concentrations, and breakthrough characteristics are presented. The results have revealed that removal of calcium, and hence hardness, through ion exchange with clinoptilolite under those circumstances is a promising alternative, with surface capacities reaching 11 mg calcium/g clinoptilolite.