A study has been made of the preparation and properties of aluminum oxide films prepared by reactive sputtering. Deposition temperature, rate, oxygen pressure, etching characteristics in , and phase changes of the deposited films as a function of heat‐treatment were investigated. Two phases of deposited alumina could be prepared; either an amorphous, soluble, or a polycrystalline form of γ‐alumina, which was considerably more resistant. On the basis of the observed changes in solubility, refractive indices, and phase changes, a tentative model is proposed which could be pertinent to catalytic behavior of alumina powders.