The Supercritical Water Reactor is one of the Generation IV nuclear power plant designs envisioned for its high thermal efficiency. Uniform corrosion is one of the main challenges to finding suitable materials for structural components and fuel cladding. The corrosion rate depends on the nature of the protective oxide formed, such that small alloying content differences cause significant differences in corrosion rate. By studying in detail the oxide layer with a combination of microbeam synchrotron radiation diffraction and fluorescence and transmission electron microscopy, it is possible to discern which characteristic oxide structures lead to protective behavior and a lower corrosion rate. A review is presented of studies conducted in protective oxide layers formed during corrosion ofzirconium alloys and in ferritic-martensitic and oxide dispersion strengthened steels in supercritical water at 500-600 C for different exposure times. Microbeam synchrotron radiation diffraction and fluorescence allows us to probe the structure of the oxide layers with unprecedented sensitivity and submicron spatial resolution. For the zirconium alloys a characteristic structure formed at the oxide-metal interface is associated with protective behavior. In the case of advanced steels, a sequence of sub layers of oxide layers is observed where particular phases are shown to be crucial to protective behavior.