Aluminum capacitor foil with a tunnel etch structure was reacted with boiling water to deposit a layer of psuedoboehmite and was then anodized to voltages between 55 and 540 V in a neutral hot berate solution. The resulting composite oxide film consists of two layers of crystalline barrier gamma'-oxide containing a high density of nanovoids, covered with a layer of residual hydrous oxide. A uniformly thin 50 nm section was prepared by focused ion beam. The specimen was cut parallel to the foil surface so the oxide shell within an etch tunnel appears in cross section. These were examined by transmission electron microscopy to reveal oxide structural features with a resolution of a few nanometers. The void structure is different in inner and outer crystalline layers, reflecting different growth mechanisms, and the void distribution changes as the film thickens. The crystalline region develops initially in the center of the anodic oxide, with a layer of amorphous anodic alumina on either side. Crystalline fronts spread toward both interfaces until all anodic alumina is transformed to gamma'-oxide and reaction with psuedoboehmite transforms that nonbarrier layer to barrier gamma'-oxide. Details of film growth and structure are different for oxides grown in etch tunnels and on a smooth substrate. The field coefficient, qa/kT, for 500 nm thick crystalline oxides is not significantly different than that for amorphous barrier oxides. (C) 2000 The Electrochemical Society. S0013-4651(00)05-083-7. All rights reserved.