The selective oxidation of three CMnSi steels with different C levels was studied by XPS. The formation of a gradually covering SiO2 film is observed until a transition temperature above which the oxide film disappears, resulting in a renewed appearance of metallic iron at the surface. This change in oxidation behavior is thought to be due to the SiO2 reduction by solute C which is thermodynamically favorable at high temperature. At the same time the precipitation of a BN film on the metallic surface was observed which prevented any further oxidation.