Pure Mg, Mg-5wt%Ca, and Mg-5wt%Ca-1wt%Zn alloys were cast, and their high-temperature oxidation behavior was investigated to determine the roles of Ca and Zn in oxidation. Ca and Zn additions refined the microstructure by formation of Mg2Ca and Ca2Mg6Zn3, respectively, at grain boundaries. Ca increased the oxidation resistance by forming a thin, compact CaO scale on the surface. However, Zn decreased the oxidation resistance because of the formation of Ca2Mg6Zn3, which increased the MgO-forming tendency. † (