The changes in the structural, morphological and acidity properties of nanocrystalline aluminas provoked by hydration under a controlled humidity atmosphere at room temperature were studied. Besides the physical and chemical adsorption of water on the surface, the hydration process causes the formation of aluminum oxy-hydroxide and trihydroxide (boehmite and bayerite). The recalcination of hydrated gamma-Al2O3 cannot recuperate the initial acidic properties of the material. The populations of weak to moderate acid sites strongly decrease under recalcination of hydrated aluminas. On the other hand, the stronger acid site populations are not strongly affected by recalcination. (c) 2006 Elsevier B.V. All rights reserved.