The evolution of iron-oxide layers at room temperature on pure polycrystalline alpha-Fe and on the Fe-N phases gamma-Fe[N], gamma’-Fe4N1-x and epsilon Fe2N1-z was followed in situ with Auger-Electron Spectroscopy. The observed oxidation kinetics of the Fe and Fe-N phases were interpreted using a model considering coupled fluxes of cations and (tunnelling) electrons and an electrostatic potential difference over the oxide layer changing during oxidation. It was concluded that the oxidation kinetics are largely controlled by cation transport governed by the chemical and electrostatic potential differences.