Al2O3 layers were grown by anodizing Al films sputtered on borophosphosilicate glass/SiO2/Si substrates and subsequently characterized by transmission electron microscopy, and by current-voltage and capacitance-voltage measurements using a metal/insulator/metal structure. Amorphous anodic oxide is formed on nonpreannealed Al film but a complex oxide film generally consisting of five sublayers (designated as amorphous I, porous I, crystalline, porous II, and amorphous II) is formed on preannealed Al film. Freshly formed anodic oxide is always amorphous in both preannealed and nonpreannealed Al thin films. The growth of the crystalline layer in preannealed samples is due to the amorphous-to-crystalline transition of the amorphous II layer. The porous I layer is formed well before the crystalline layer. The voids in this layer are probably due to the trapping of oxygen at the sites of crystalline oxide. The voids in the porous II layer are small and their formation can be attributed to the volume shrinkage associated with the amorphous-to-crystalline transition of the amorphous II layer. The amorphous oxide films exhibit excellent dielectric properties. The dielectric properties of the complex oxide are much inferior and vary significantly due to structural variation.