A series of Fe-15Cr-(2-3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M 23 C 6 precipitates. The alloys were oxidized in the austenitic state at 850 °C in pure oxygen, with and without a pre-oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia-rich scaling took place if the chromium concentration at the metal-scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre-oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide-free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.