Chomia forming steels are potential candidates for the use as low cost interconnectors in solid oxide fuel cells (SOFC) for electric power generation. The materials however, are subjected to humid air at high temperatures up to 800degreesC leading to an enhanced oxidation. High temperature X-ray diffraction has been applied to study the oxidation of Nicrofer 45TM and F17TNb in dry air and in humid air. Nicrofer 45TM forms a coherent Cr2O3 scale in dry air. The Cr2O3 scale is thicker in humid air and additionally MnCr2O4 is observed as well as internal SiO2 formation. The F17TNb forms Cr2O3 and MnCr2O4 in both atmospheres, with a higher rate constant in humid air. The results from the in situ X-ray diffraction indicate, for both of the alloys, alternately partial spallation and rehealing of the oxide scale.