Characterization of diffusion properties of atoms in oxides is crucial for understanding oxidation mechanism of metals because the oxidation is mediated by diffusion. Employing in situ photoemission spectroscopy, we have observed diffusion-induced compositional variations of stainless-steel surfaces during oxidations at 350 and 550 degreesC in the oxygen partial pressure range 1 X 10(-9) -1 X 10(-5) Torr. Initially, a significant amount of metallic chromium was present at the surface of stainless-steel oxide layers. During subsequent oxidation, thin oxide films grew on top of the oxide layer, but there existed critical pressures below which the oxide film formed was almost complete Cr2O3. From the critical pressures and growth rates of the oxide film, diffusion rates for chromium through the oxide layer have been determined and a mechanism for the diffusion of chromium in the stainless-steel oxide layer is proposed.