A study of the oxidation and magnesium evaporation kinetics has been carried out on an aluminum−3% magnesium alloy. A metallographic examination of the surface oxide films produced was included. The alloy was oxidation resistant to 200°C. At temperatures greater than 350°C the oxidation rate was initially inversely proportional to the oxide film thickness, transforming to a constant rate for long exposures. Selective oxidation of magnesium caused formation of aluminum inclusions in the surface oxide. These inclusions imparted a black discoloration to the alloy surface. Magnesium evaporated from the alloy in a vacuum at temperatures greater than 350°C after an induction period, the duration of which was dependent on the temperature. A comparison of the magnesium evaporation and alloy oxidation rates demonstrated that the oxide film offered resistance to oxidation of the metal.