A chromium coating of 25μm depth was physically deposited on two iron substrates differing mainly in their respective carbon content: pure iron and iron containing 0.3 wt% of carbon (Fe0.3C). After oxidation, the oxidized layer was thin and consisted mainly of Cr2O3. In the case of Cr on Fe0.3C, carbon was located in the deeper part of the oxide, and A1 showed a strong enrichment both in the very outer part of the oxide scale and in its inner zone. With the help of successive oxidations in16O2, then18O2, it was observed that the carbon-enriched zone in the oxide layer on the Cr coating of the Fe0.3C sample behaves as a good barrier against oxygen inner diffusion. On pure iron, the carbon-enriched layer does not prevent oxygen diffusion into the Cr coating and both a cationic and an anionic mechanism occur for the growth of the oxide layer. The oxygen diffusion parameter in Cr2O3is the same for the two substrates: 10−14cm2s−1.