Oxide films formed naturally on pure Mg are investigated by the use of ultramicrotomed cross sections and transmission electron microscopy. The film formed in air immediately after scratching the metal surface is initially thin, dense, amorphous, and relatively dehydrated. Continuing exposure to humid air or exposure to water leads to the formation of a thicker hydrated film adjacent to the metal. The film formed in water contains an additional toy! layer characterized by a plateletlike morphology. The structure of these layers and their significance in corrosion protection are discussed. The changes occurring in these structures as a result of exposure to the electron beam are reported.