AbstractPrevious investigations show the protective nature of CaO on Mg and commercial Mg alloys during high temperature oxidation. The addition of CaO to Mg alloys increases the health and safety at workplaces and the quality of the cast surface by increasing the oxidation resistance. However, the role the Ca or CaO on influencing the oxidation resistance is not clear. To understand the effect of Ca on the oxidation behaviour we focused on pure Mg and the investigation is conducted on Mg and Mg-CaO castings and their interaction with and without Ar-SF6 as cover gas during the casting process. The oxide layers are investigated with SEM and TEM to characterise the role of Ca and CaO.