The structure and properties of anodic Al2O3 films formed at different anodizing temperatures from thin Al films deposited by magnetron sputtering on oxidized Si wafers have been studied by transmission electron microscopy, secondary ion mass spectrometry, and electrical measurements. The anodic oxide films formed at 8 degrees C and 25 degrees C are amorphous with excellent dielectic properties, leakage current lower than 500 nA at 7.5 MV cm(-1) and no breakdown up to 7.5 MV cm(-1). Partially crystalline gamma’-Al2O3 is grown at the upper half of the 58 degrees C oxide film. This film exhibits much inferior dielectric properties than the lower temperature films. Carbon is incorporated into the top portion of the oxide films from the electrolyte during anodizing and it has little effect on the crystalline oxide formation. Oxygen penetrates into the underlying Al layer during anodizing. The penetration increases with increasing anodizing temperature.