The formation and oxidation of magnesium-aluminum oxynitride (MgAlON) were investigated with emphasis on the effect of impurities through using synthetic raw materials and natural raw materials. It was observed that the formation of MgAlON was not much influenced by the starting materials, but MgAlON prepared from the natural starting materials exhibited accelerated oxidation. The different oxidation behaviors of MgAlON prepared from the different raw materials were discussed from the viewpoint of impurities in the materials. The formation temperature and oxidation temperature of MgAlON produced in this work were compared with those of MgAlON prepared using other processes.