Korean natural zeolite in which clinoptilolite and mordenite coexisted with feldspar and illite as impurities, was treated with 1.0, 3.0, and 5.0 M NaOH solutions at 100, 150, and 200degreesC under autogeneous pressure for 17 hours either with or without an air classification as pretreatment. Phillipsite, analcime, and hydroxycancrinite were identified as reaction products depending on the reaction temperature and NaOH concentration. The air classification of the starting material prior to alkali-hydrothermal treatment effectively reduced the amount of feldspar, which hardly reacted to zeolite in the hydrothermal reaction. The ammonium adsorption behavior of the treated and untreated samples were investigated in solutions of between 10(-3) M and 10(-2) M NH4Cl. The amount of adsorbed ammonium ions in alkalihydrothermally treated product from air-classified material was higher by about two times than was that of corresponding untreated zeolites. The air-classified zeolite treated in 3 M NaOH solution at 100degreesC showed the highest adsorption of ammonium ion among samples. It was explained by both the phase change of clinoptilolite and mordenite to phillipsite with higher cation exchange capacity and the reduction in the amount of feldspar that was less reactive under hydrothermal conditions for the formation of phillipsite. The results indicated that the combination of the air classification and alkali-hydrothermal treatment effectively improved the adsorption behavior for ammonium ions on natural zeolites with impurities.