Clinoptilolite has been widely studied for ammonium removal in the past 2 yr. However, many investigators have reported variations in the measured capacities of samples of clinoptilolite. These studies and the factors believed to influence measured zeolite capacity are reviewed. In addition no studies to evaluate other natural zeolites for ammonium removal have been reported. In this study samples of clinoptilolite, erionite, mordenite and phillipsite provided by the Anaconda Company were evaluated for ammonium removal from wastewaters. In addition, samples of clinoptilolite were pretreated in various ways to determine whether an improvement in ammonium removal performance could be realized. Total exchange capacities, capacities for ammonium removal from a synthetic waste, packed bed densities and crushing strengths were measured. Phillipsite was found to have almost twice the weight capacity for ammonium removal from synthetic waste compared to that of clinoptilolite. The volumetric capacity was 26% better than that of clinoptilolite. However, the phillipsite sample was extremely friable and it could not be used for water treatment unless it was strengthened with a binder. Pretreatment of clinoptilolite with NaOH, HNO 3 and steam did little to improve the zeolite's performance. However, heat pretreatment (600°C for 1 h) improved the zeolite's selectivity for ammonium significantly. Ammonium removal capacities were increased by approximately 17% for heat treated zeolite samples although the total exchange capacity of the zeolite was reduced somewhat.