Two distinct iron substrates (pure iron and iron containing 0.3 wt% of carbon) have been protected by chromium using physical vapor deposition. The oxidation behaviour of the Cr coating in oxygen at 700 °C depends on the iron grade, due to the influence of carbon. The study shows that carbon can be dissipated into the surrounding atmosphere, depending on its stability in the substrate and on the nature of the atmosphere (confined or dynamic oxidizing atmosphere). Carbide precipitation in the coating, near the oxide interface, decreases the oxidation rate. There is no large influence of the nature of the substrate on oxygen diffusion into the outer part of the Cr 2 O 3 layer. Oxygen diffuses preferentially via interfaces, with a diffusion coefficient of 10 - 1 4 cm 2 s - 1 . In the chromium coating, there is a significant difference in oxygen penetration, depending on the grade of iron. Carbon incorporation into the coating is greater with the pure iron substrate than with the substrate made of iron containing 0.3 wt% carbon, and the presence of carbon in the coating decreases the oxygen diffusion, particularly along grain-boundaries : thus D g b (O→Cr = 6.2 10 - 1 3 cm 2 s - 1 and D g b (O→Cr + C) = 2.2 10 - 1 2 cm 2 s - 1 .