In the present work, moisture absorption of an original expanded Epoxy/dicyandiamide solid formulation designed for the automotive industry was studied. The first part of work focuses on the bulk of cured adhesive joints, according to the water absorption under hydrothermal aging. Aging in immersion in distilled water at 40 degrees C, associated to gravimetric, InfraRed, and optical microscope analyses, enables to estimate the coefficient of diffusion of the formulation in the considered conditions and structural modifications induced by the aging. The studied formulation is stable during the aging, as the structural adhesive network does not degrade, even after 1500 h of immersion. The lixiviation of some compounds of the formulation eventually occurs. The water uptake is always reversible and does not strongly impact the integrity of the joint, even in the case of a fully expanded adhesive joint. The hygrothermal aging enables to define three distinct diffusion domains, and to estimate the coefficient of diffusion of the adhesive joint. Results obtained in this study can be used to estimate the durability of a metal/adhesive interface in hygrothermal aging conditions. As the expanded adhesive joint does not seem to be degraded during the aging, the integrity of the joint is supposed to essentially depend on the durability of the interface and the corrosion of the metal substrate.