This study presents basic research on the use of natural zeolites for the removal of ammonium ions from aqueous solutions. Three natural zeolites from Shimane, Japan, used in this study are shown to contain varying amounts of clinoptilolite and mordenite. The chemical behavior of these natural zeolites was investigated in distilled water, hydrochloric acid, and ammonium chloride solution. The amount of ammonium ions adsorbed by these zeolites was measured using an electrode specific to ammonium ion. The results were compared with published adsorption values for other zeolites. Hydrogen ions displaced the exchangeable cations (Na+, K+, Ca2+, and Mg2+) on the natural zeolites in distilled water and hydrochloric acid. With the decrease in equilibrium pH, the amounts of dissolved silica and alumina increased. This behavior indicates that the structures of the zeolites are damaged under this condition. Ammonium ions were preferentially adsorbed on the natural zeolites in ammonium chloride solution. The amount of ammonium ions adsorbed quantitatively corresponded to the theoretically expected amount at high ammonium concentrations. This result indicates that ammonium adsorption occurs by ion exchange of ammonium ions with Na+, K+, Mg2+, and Ca2+ in cation exchange sites on zeolites. The order of selectivity of ammonium ions for other cations on the zeolites was Na+ >> K+ > Ca2+ > Mg2+. Ammonium adsorption was the highest in the zeolite containing the highest proportion of clinoptilolite of the three natural zeolites. The maximum adsorption was 1.28 mmol/g.