Weight changes in Al-2 and 3.5 mass% Mg alloy samples were measured by thermogravimetric analysis. X-ray diffractometer tests were used to investigate the progressive development of thermally formed oxide films on the samples. Both samples were first heated in a dry air atmosphere. The oxide film formed on the Al-2 mass% sample comprised -alumina, MgO, MgAl 2 O 4 , and gibbsite. The film formed on the Al-3.5 mass% sample contained large quantities of MgO, but no MgAl 2 O 4 .The samples were then heated in a nitrogen gas atmosphere for <1:9 h. The Al-3.5 mass% sample contained greater amounts of gibbsite and -alumina than did the Al-2 mass% sample. The latter yielded a greater amount of AlN (and/or MgO). After an extended holding time ($6 h), the Al-3.5 sample contained a greater amount of AlN (and/or MgO), and its weight increased remarkably. The as-cast samples containing the Al-3.5 mass%Mg cube had a higher percentage of foggy film compared to those containing an Al-2 mass%Mg cube. Oxide films that are readily formed during melting tend to be trapped in aluminum alloy castings.