Surface oxide microstructures have been examined on composites prepared by directed melt oxidation of Al-Mg-based alloys. Evidence has been found that the pick-up of oxygen occurs by vapour-phase oxidation of magnesium to magnesium oxide, which is subsequently reduced to form alumina in a cyclic process. It is shown that other volatile elements, such as lithium, sodium and zinc result in the formation of solute-rich oxides whose stability may also be brought about by vapour enrichment. In all the above cases it was found that alumina matrix composites form beneath the surface (though the magnitude is small in the case of sodium). A comparison is made of the different types of oxidation behaviour among various systems that contain volatile solutes, i.e. Al-Mg, Al-Zn, AI-LI and Al-Na.