The influence of pretreatment (ion bombardment) on the initial oxidation of AISI 314 stainless steel has been investigated in ultra high vacuum systems with ellipsometry and Auger electron spectroscopy. The bombardment was carried out with Ar+ ions with energies up to 2.2 keV at various doses and angles of incidence. Oxidation was performed at low Pos (< 10−5 Torr) and sample temperatures (<300°C). The worked layer was removed with low-energy argon ions. High energy bombardment causes an increase in oxidation rate and higher values of the real and imaginary part of the refractive index as compared to the undisturbed sample. The rate and extent of oxidation increases with increasing energy, dose and angle of incidence of the ion beam. The outermost part of the oxide is enriched in iron while the region near the substrate contains more nickel and chromium.