In the present work the development of residual strains in the iron-oxide layers growing on alpha-Fe and phosphated alpha-Fe at 400degreesC in artificial air at 1. arm was investigated by X-ray diffraction of synchrotron radiation, both in-situ during oxide growth and at room temperature after cooling. The oxidation kinetics of alpha-Fe and phosphated alpha-Fe at 400degreesC and the microstructural state of the oxide layers are first presented. Then a detailed study of the residual strains in the oxides layers is undertaken. Correlations between the residual stresses measurements and the successive parabolic oxidation stages for phosphated alpha-iron are established. It leads to a better understanding of the oxidation behaviour.