The surface of an aluminium alloy powder containing 2% of magnesium was examined by X-ray photoelectron spectroscopy (XPS) before and after annealing in various atmospheres, air, vacuum and argon. It was found that the evolution of the surface composition depends on the nature of the gaseous atmosphere. In open air the surface became covered with a layer of magnesium oxide which prevented the alloy powder from zinc sublimation. In vacuum no zinc enrichment was observed whereas its volatilization was ascertained by other methods. Under flowing argon an enrichment in magnesium oxide and also in zinc was observed.