Corrosion tests were performed in steam and supercritical water at 500 °C for two ferritic–martensitic alloys: HCM12A and NF616. The corrosion kinetics for the two alloys are similar in both environments showing near cubic behavior, but the corrosion rate was significantly higher in supercritical water than in steam. Examinations of the oxide layers using scanning electron microscopy and microbeam synchrotron diffraction and fluorescence show that both alloys form two-layer oxide structures in either environment. The outer layer contains only Fe₃O₄, while the inner layer contains a mixture of Fe₃O₄ and FeCr₂O₄. Additionally, marker experiments using a novel photolithographic deposition process show that the original water–metal interface corresponds with the outer–inner layer interface as expected. The results are discussed in light of known corrosion mechanisms.