Hydration of an aluminum oxide surface under an adhesive film was shown to occur when the specimen was immersed for extended periods in hot water. The absence of a crack and lack of access of the water to the specimen edges forced the moisture to reach the interface by migrating through the adhesive. The increased volume of the hydration product forced it to erupt through the adhesive. This subadhesive hydration is conclusive proof that hydration can proceed in advance of a crack to promote its propagation. Electrochemical impedance spectroscopy (EIS) measurements on the specimen during exposure readily detected both the ingress of moisture through the adhesive and the hydration of the substrate.