Chromium, oxygen and iron ion diffusivities were determined in thermally grown natural chromia layer on a model Fe-15 wt.%Cr alloy in the temperature range from 750 degrees C to 900 degrees C, in air atmosphere. The stable isotopes O-18, Cr-54 and Fe-57 were used as oxygen, chromium and iron tracers, respectively, and the diffusion profiles were established by secondary ion mass spectrometry (SIMS). Oxygen bulk, effective and grain boundary diffusivities are lower than the corresponding chromium or iron diffusivities, while the iron diffusivities are greater than the chromium ones. Grain boundary is a fast path for chromium, iron and oxygen ion diffusions in natural chromia layer grown on the Fe-15%Cr alloy. Therefore, the oxidation rate of the Fe-Cr alloy is mainly controlled by grain boundary ion diffusion. The values of the calculated parabolic oxidation constant, according to Wagner's theory, are close to the experimental ones, assuming that chromium and oxygen ion diffusions in chromia grown on the Fe-15%Cr alloy occur in an extrinsic regime. Moreover, chromium ion diffusion plays the main role on the oxidation of the Fe-15%Cr being sufficiently large to maintain the oxidation rate of the alloy. (C) 2015 Elsevier B.V. All rights reserved.