The goal of this paper is to illustrate how in situ X-ray absorption near edge structure (XANES) investigations can give a more detailed understanding of the function of various parameters on the mechanisms of passivity. In situ XANES has been used to monitor changes in oxidation state under electrochemical control that lead to a solid-state conversion or dissolution of metal or oxide. Recent studies on native and artificial passive films-mainly Fe, Cr and Fe-Cr alloys, are employed for this purpose.