The effects of heat-treatment (before and after anodization) on the microstructure and electrochemical characteristics of anodized aluminum oxide films formed in 85degreesC aqueous ammonium adipate electrolyte were investigated. The morphology and crystal structure of the anodized oxide were examined by transmission electron microscopy. The capacitance, relative dielectric constant, electrochemical impedance spectroscopy, and I-V behavior of the oxide film were also determined. Both pre- and post-heat-treatment at 500degreesC could induce the formation of crystalline gamma'-Al2O3 in the outer layer of anodized oxide and consequently increase the relative dielectric constant of the film. The differences in the morphology and crystalline characteristics between the anodized oxide subjected to pre- and/or post-heat-treatments led to variations in the electrochemical properties. The pre-heat-treatment could economize the required charge to anodize the oxide and retard the growth of film thickness during anodizing. Thermal-induced phase transformation from amorphous to crystalline oxide of the anodized film due to post-heat-treatment, could leave some defects and substantially decrease the electrical resistance of the oxide layer. The reanodization further developed and extended the crystalline oxide formation, which subsequently increased the relative dielectric constant of the oxide film. (C) 2004 The Electrochemical Society.