In this study, the Chinese natural clinoptilolite (sample 1) was fused with sodium hydroxide prior to hydrothermal reaction, and it was transformed to modified zeolite Na-Y (sample 2). The uptake of ammonium ion from aqueous solutions in the concentration range 50-250 mg NH(4)(+)/l on to the two samples was compared and the equilibrium isotherms have been got. The influence of other cations present in water upon the ammonia uptake was also determined. The cations studied were potassium, calcium and magnesium. In all cases the anionic counterion present was chloride. The results showed that sample 2 exhibited much higher uptake capacity compared with sample 1. At the initial concentration of 250 mg NH(4)(+)/l, the ammonium ion uptake value of sample 2 was 19.29 mg NH(4)(+)g(-1) adsorbent, while sample 1 was only 10.49 mg NH(4)(+)g(-1) adsorbent. For the natural clinoptilolite, the effect of the metal ions suggested an order of preference K(+)>Ca(2+)>Mg(2+). These contrasted with the modified zeolite, where the order appeared to be Mg(2+)>Ca(2+)>K(+).