Abstract Pure Mg was oxidized in 20 kPa O 2 , at temperatures between room temperature and 400°C, for times ranging from 2 min to 60 h. The oxide layer was analysed by XPS. The samples were pre‐treated by mechanical polishing and ion etching. Oxidation was carried out in a furnace connected to the XPS instrument. Magnesium was also oxidized in CO 2 at 150°C and in water in order to gain information about the binding energies of the elements in MgCO 3 and Mg(OH) 2 . The binding energies of Mg 2p, O 1s and C 1s of these compounds, as well as MgO, are reported. The thickness of the oxides formed during oxidation for 15 min at room temperature and at 150°C are 1.5 and 1.7 nm, respectively. At 300°C the growth of the oxide is well fitted by an inverse logarithmic law. The fact that the oxide formed in a dry atmosphere up to 300°C is extremely thin indicates that it has a low defect density. The influence of the humidity on the corrosion properties of Mg is discussed. The influence of the high vapour pressure of Mg on its oxidation properties is also mentioned. Copyright © 2002 John Wiley & Sons, Ltd.