The mechanisms of plasma post-oxidation of plasma nitrided AISI 1045 plain steel were investigated. The influence of plasma post-oxidation temperature and time on the oxide layer thickness, morphology, and composition were addressed. The oxide thickness grows exponentially with temperature, with activation energy of 68±5kJmol −1. The time dependence of the oxide layer thickness, on the other hand, is governed by a diffusion-reaction process. It was verified that temperature plays an important role on the morphology of the oxide. Indeed, at the highest temperature, 550°C, the oxide layer is not homogeneous and has a lower hardness than oxide layers obtained at 480 to 500°C. The latter seem to be more favorable temperatures to grow compact, homogeneous, and harder oxide layers. The oxide–nitride bi-layer produced here contains a mixture of γ′-Fe4N and ε-Fe2-3N and only one iron oxide, Fe3O4 (magnetite). The proportions between these phases vary with the plasma processing temperature and time. ►Oxygen and nitrogen profiles were determined by alpha-particle resonant scattering. ►The oxide thickness as a function of time does not follow a pure parabolic law. ►The oxidation process is governed by a diffusion-reaction mechanism.