The hydration reactions (in boiling water) of aluminum oxide films, which are used in electrolytic capacitor applications, are investigated. The films formed by a two-step anodization process in various electrolytes at 100 V are investigated. The anodization solutions are ammonium adipate, phosphoric acid, ammonium dihydrogen phosphate, and their mixtures. The cross-section morphologies and crystal structure of the oxides are examined by transmission electron microscopy. The hydration resistance of the anodized film, judging by its variation in film capacitance and retention (tolerance) voltage after immersion in boiling water for 15 min, is explored. After the hydration test, an outer fibrous hydrated layer was observed for each film primarily anodized in ammonium adipate (regardless of the electrolyte used in the re-anodization process). The hydrated layer is a composite with many fine gamma'-Al2O3 grains dispersed in the pseudoboebmite matrix. Re-anodization in phosphorous-containing electrolytes can cause an increase in hydration resistance by retarding the formation of fibrous hydrated oxide. Phosphorus is found to be incorporated in the oxides for those formed in phosphorous-containing primary anodization electrolytes. The presence of phosphorus in the oxides can significantly improve their stability against hydration. The formation of fibrous hydrated layer results in an increase in capacitance and a decrease of retention voltage. (C) 2004 Elsevier B.V. All rights reserved.