Ultramicrotomed sectional specimens of composite aluminum oxide films, formed by the sequential process of the reaction with boiling water followed by anodic oxidation, were examined using transmission electron microscopy. It was found that hydrous oxide films, the structure of which is indicated to be pseudoboehmite from infrared spectra, consist of a dense inner layer of relatively uniform thickness and a fibrous outer layer of uneven thickness. TEM observation showed that the composite oxide films consist of two layers-namely an outer crystalline oxide layer containing numerous voids and cracks, which was identified by electron diffraction as γ′-Al₂O₃ and an inner amorphous oxide layer. During anodizing, the total thickness of the composite oxide film increased with time, while the thickness of the inner dense layer of hydrous oxide film decreased linearly with time and disappeared after a certain period t_d. The thickness of the outer fibrous layer of the film remained almost unchanged during t_d and then decreased gradually. The electric field supported by the composite oxide film obtained by immersion in boiling distilled water for 15min and subsequently anodizing at 1mA/cm² to 200V in 0.27MH₃BO₃/0.0092M (NH₄)₂O· 5B₂O₃·8H₂O solution was found to be 12×10⁶V/cm, which was somewhat higher when contrasted with the values in the range 10∼11×10⁶V/cm obtained by testing several aluminum electrolytic capacitors for commercial use.