Abstract: Natural zeolite minerals which have a high silicon-to aluminum atomic ratio (hereinafter referred to as Si/Al ratio)and a fibrous micro-structure in at least some parts are ore-dressed and classified according to the iron impurity content converted in terms of iron oxide Fe.sub.2 O.sub.3. Zeolite with high Si/Al ratio is excellent in resistance to acid and heat and can be re-activated at high temperatures. The fibrous microstructure is considered to impart a high degree of mechanical strength to the zeolite after pelletization. High-grade natural zeolites with small iron impurity content uniformly exhibit high adsorption performance.In particular, zeolite adsorbents using natural zeolite consisting principally of mordenite are very useful for various purposes.In the case of removing oxygen, the combined use of the zeolite adsorbent and an oxygen removing catalyst provides good results. By cooling the zeolite adsorbent down to low temperatures, its adsorption performance rises sharply.