The stability of y-alumina suspensions in water has been investigated by long-duration experiments (1 day to 6 months). Several complementary methods were used to characterize the solid (acid-base titration and dissolution rate measurements, X-ray photoelectron spectroscopy, X-ray diffraction, gravimetric/differential thermal analysis, infrared spectroscopy, and scanning electron microscopy). It appears that y-alumina, which is widely used as a model oxide in sorption experiments, thanks mainly to its high specific surface area and to the presence of aluminol groups on its surface, is progressively transformed to bayerite (beta-Al(OH)(3)).' This transformation was characterized by an induction period of about 4 days, corresponding to the formation, of a transient amorphous hydrated phase, followed by an increase in the bayerite concentration, which levels off after about 2 months. This transformation results in a dramatic decrease in the surface density of sites reactive to protons. Thermodynamical calculations predict the hydration reaction of y-alumina leading to a more stable phase (bayerite, gibbsite, or boebmite). Since the surface reactivity and sorption properties of solids are factors controlling the transport of elements in water, it is necessary to take into account the stability of solids when modeling sorption experiments. These factors are important for the long-term prediction of the effectiveness of barriers placed around radioactive waste depositories.