Morphology and structure of oxide films formed naturally on magnesium-aluminum alloys by exposure to humid air and water have been investigated by use of ultramicrotomed cross sections by transmission electron microscopy and compared to the properties of films formed on pure magnesium. Similar to pure magnesium, the initial air-formed film is amorphous and dense. The film formed in water has a three-layered structure consisting of a hydrated inner layer, a dense intermediate layer similar in structure to the air-formed film, and a top layer with platelet-like morphology. With increasing aluminum content of the alloy, all layers become dehydrated and enriched in aluminum oxide. Furthermore. the innermost and outermost layers decrease in thickness. These changes are particularly significant when the aluminum content of the alloy is increased above 4%, which corresponds to about 35% Al in the innermost layer. This threshold is characterized by a significant improvement in the corrosion resistance. The improvement is attributed to the alumina component of this inner Layer becoming the dominating : factor in determining the passivity of the surface, presumably by forming a continuous skeletal structure in an amorphous mixture of aluminum and magnesium (hydr-)oxides.