Plasma oxidation of thin polycrystalline aluminum films (∼100 A˚) on GaAs has shown that oxidation of aluminum proceeds by an initial rapid grain‐boundary oxidation of the aluminum followed by a slower oxidation towards the centers of the individual grains. Continued oxidation results in the growth of Ga‐As‐ oxide layers on both sides of the Al‐oxide film indicating that the oxidation of GaAs proceeds by an electric‐field‐assisted in‐migration of oxygen through the Al‐oxide layer toward the interface and the out‐migration of Ga and As toward the surface. This oxidation process can be used to form an amorphous film of Al‐oxide on GaAs. By using the Al‐oxide film as a preferential filter for the migration of Ga and As, an amorphous Ga‐As‐oxide film with a Ga/As concentration ratio ofunityall the way to the GaAs substrate can also be achieved.