Films formed on superpurity aluminum and certain of its binary alloys were studied employing anodic polarization and electron transmission microscopy. Specimens were prepared by electropolishing followed by either oxidation in dry oxygen, anodic oxidation, or anodic oxidation followed by high‐temperature annealing. The nucleation and growth of gamma alumina crystallites during oxidation at 450°C and the inhibiting effect of copper and particularly silicon on this process was explained by assuming that an 0.001% magnesium impurity was responsible for the nucleation. The change in ionic conduction of anodic oxide films on annealing was interpreted as arising primarily from a decrease in the number of aluminum ions which can become current carriers, rather than a change in activation energy or activation distance. Annealed anodic films and “amorphous” films formed in dry oxygen could not be distinguished. The observations in general, are consistent with Wilsdorf's model for the “amorphous” oxide films, randomly oriented “molecular” groups.