Results obtained by low‐energy electron‐diffraction and photoelectric work‐function measurements indicate that the following features result from exposure to oxygen at room temperature. Initial adsorption occurs in an amorphous molecular form. Some of the adsorbed molecules diffuse over the surface to lattice‐defect sites where they dissociate. The resulting atoms form in a lattice structure surrounding the defect site. With continued exposure to oxygen, a place exchange between some oxygen and nickel atoms occurs to form a single‐spaced simple‐square structure with a slight increase in the lattice constant. This place exchange immediately precedes, in exposure, the formation of the oxide structure of the rock‐salt type. The work function of clean (100) nickel depends on the type of anneal. &phgr;=5.08 ev after argon ion bombardment, &phgr;=5.12 ev after a small anneal, &phgr;=5.22 ev after a more complete anneal.