Uniform corrosion is a major concern for ferritic-martensitic steels when considered as candidate materials for the supercritical water reactor (SCWR). The corrosion rate depends on alloy composition and microstructure. The corrosion rate depends on alloy composition and microstructure. The best ferritic-martensitic alloys resist corrosion by developing a protective oxide layer that stabilizes oxide growth. To better understand the protection and stabilization mechanism, the structure of oxide layers formed on ferritic-martensitic alloys in supercritical water is studied using both transmission electron microscopy and microbeam synchrotron radiation diffraction and fluorescence. Using the microbeam it is possible to determine phases present using x-ray diffraction and chemical composition using x-ray fluorescence, both as a function of location in the oxide layer. The detailed study of phases present and elemental segregation at interfaces is presented for an oxide formed on 9Cr ODS steel after exposure to supercritical water for 667 hours. In the diffusion layer, both metal and oxide peaks are seen indicating a coexistence of the two phases in the diffusion layer. In the Cr-rich inner layer, a mixture of spinel phase FeCr2O4 and Fe3O 4 is observed, while in the outer oxide layer, Fe3O 4 is the predominant phase. Evidence for additional Cr-rich phases near the interfaces is also shown.