Insulating films of aluminum oxide have been prepared by evaporation of aluminum in a partial pressure of 10−3 mm Hg of oxygen. On exposure to air both dielectric constant and dissipation factor increase to values similar to those observed for anodized films on bulk aluminum. The results can be understood if the initial insulating film is a suboxide similar to the postulated which is normally unstable at room temperature in air or oxygen, decomposing to .