The adsorption of atomic oxygen on the unmodified and I-modified Ag(100) surface has been studied with the density functional theory method. The various mechanisms of iodine atom modification to affect the adsorption of atomic oxygen have been investigated. The most stable adsorption site for both oxygen and iodine atom on the silver surface is the fourfold hollow site. The modification of iodine atom changes the electronic properties of the substrate and strengthens the bond between the oxygen atom and the surface. The distance from the subsurface oxygen to the surface on the I-modified silver surface is smaller than that on the clean surface. These all benefit the enhancement of the selectivity and the reactivity of the partial oxidation of methanol. The formation of the subsurface oxygen at both the clean and I-modified surface is also investigated. The introduction of the iodine atoms has little effect on the penetrative process. The energy barrier for the penetration of the surface oxygen atoms is about 2.1 eV, which is less than the adsorption energy. (C) 2003 American Institute of Physics.