The adsorption of oxygen on the clean, chlorine-modified Ag(l 11) surface has been investigated by the ab initio pseudopotential total energy method based on density functional theory. The effects of chlorine on the adsorption of oxygen and the diffusion of surface oxygen have been studied. From the theoretical adsorption energies and structures of the adsorbate-surface system, we found that the existence of chlorine increases the adsorption energy of oxygen. Since the adsorbed chlorine atom will occupy the stable adsorption site for the oxygen atom, it will lead to the reduced adsorption of oxygen. We have calculated the barrier for the diffusion of atomic oxygen on the silver surfaces, and the results show that the diffusion of the surface oxygen atoms into bulk Ag is easier on the chlorine-modified Ag(l 11) surface than on the clean surface.