Using density-functional calculations, we have studied the adsorption of atomic oxygen on the Fe(0 0 1) and Fe(1 1 0) surfaces with and without Mn substitution at the top layer, as well as the diffusion mechanisms of oxygen and metal adatoms (Fe, Mn) on these two surfaces. The elementary processes studied give microscopic insight into the first stages of selective oxidation of Fe-Mn steels, a phenomenon that leads, in suitable thermodynamic conditions, to the growth of MnO particles at the iron surface. Our calculations provide significant differences between the two terminations, especially in the diffusion barriers of the different atomic chemical species involved in these complex growth processes. (C) 2012 Elsevier B.V. All rights reserved.