The effect of a hydrated oxide on the kinetics of anodic oxidation of aluminum has been studied and the properties of the resulting composite oxides compared with those of purely anodic oxides. The presence of a hydrate layer reduced the amount of anodic oxide needed to support a given voltage and increased the current efficiency to as much as 100%. Water was lost from the oxide during anodization, probably as the result of transformation of boehmite to , which was incorporated into the barrier layer. The dielectric strength was greater for a composite oxide than for an oxide produced in the absence of a hydrate, though the dielectric constants were probably not very different. The impedance characteristics of a composite oxide indicated fewer microfissures than in a purely anodic oxide.