In the work which follows, the changes in structure of natural and electrolytic films brought about by various treatments will be the subject under discussion, and the experimental account will be concerned with the report of some effects encountered in the investigation of these oxide films. The confused and sometimes contradictory nature of the existing evidence regarding the nature and behavior of oxide films on aluminum is considered as sufficient justification for further study of the subject. It is hoped that some of the contradictory results obtained by other investigators may be explained on the basis of their experimental conditions and the interpretation of the data obtained.