The adsorption of oxygen(O₂) and the effect of oxygen on the chemisorption of water(D₂O) and hydrogen(D₂) on polycrystalline silver surface have been studied using the technique of thermal desorption spectroscopy(TDS) under ultra-high vacuum(UHV) condition. Three different states of adsorbed oxygen are observed on the surface. The adsorptions and reactions of water and hydrogen on the oxygen-dosed surface are strongly affected by the pre-adsorbed oxygen. Water on the oxygen-dosed surface, hydrogen is rapidly oxidized by a stepwise reaction in which hydrogen reacts first with O(a) to yield hydroxyl intermediate[OD(a)] and then this intermediate reacts to form water. The rate-determining step in this reaction is desorption of water.