The relationship between the structure, electric, and electrochemical properties of anodic passive films on stainless steels and their ability to protect the underlying metal from different types of corrosion is still a matter of considerable debate.In recent years, advanced in situ surface analytical techniques such as X-ray absorption near-edge spectroscopy (XANES), X-ray diffraction, and X-ray scattering techniques as well as scanning tunneling microscopy (STM) have made possible a much deeper insight into the structure of the anodic film on iron and iron-chromium alloys. 1-7 Using these techniques it has been shown that anodic films have nanocrystalline structure, which is to a certain extent analogous to that of the corresponding bulk oxides of iron and chromium.Concerning pure iron, it has been demonstrated that the passive film has a spinel structure which is intermediate between magnetite (Fe 3 O 4 ) and maghemite (␥-Fe 2 O 3 ). The concentration of interstitial Fe 2ϩ species in the octahedral positions of this spinel was suggested to vary from ca. 20% at low potentials to below 10% at high potentials, while in Fe 3 O 4 it is 33%. 2,3,8 It was considered likely that there is a coupling between the motion of tetrahedral vacancies and octahedral interstitials, because the interstitial cations have a high probability of occupying sites near tetrahedral vacancies. 3 It was also proposed 3 that metal interstitials are the predominating defects close to the metal/film interface, while metal vacancies predominate closer to the oxide/solution interface, since iron species are probably in a higher oxidation state closer to the electrolyte.Concerning iron-based alloys and stainless steels in particular, the strong enrichment of the passive films in Cr with respect to the base material has been recently quantified with XANES. 2 It was also demonstrated that the higher the potential is, the smaller is the enrichment in Cr of the overall passive film. The high content of Cr in the films formed on Fe-Cr alloys suggests that the structure of these films should be to some extent similar to that of the films formed on pure Cr. A supplementary view has been lent by ex situ and in situ STM studies on the structure of the crystalline oxide films on Fe-Cr alloys. They pointed to a slightly distorted triangular lattice with symmetry close to the rhombohedral (111) plane of Cr 2 O 3 , which suggests the presence of oxygen vacancies in the oxide structure. [5][6][7] In addition, sputtered thin iron and chromium oxide films on inert substrates as model systems have proven to be an important tool to assess the structure and to characterize the oxidative/reductive dissolution of passive films. 8-11 Using these synthetic oxide films, it has been demonstrated that the dissolution of the film on iron in the passive state is essentially a chemical process. The rate of which is almost independent of potential. 10 On the other hand, reductive dissolution of iron oxide and mixed iron-chromium oxides during the passive-to-active transition...