Calorimetric and X-ray diffraction investigations of anhydrous a trehalose obtained by dehydration of its dehydrate form have been performed. The results show chat the structural and thermodynamic properties of the dehydrated state strongly depend on the kinetics of water loss. A fast dehydration promotes the formation of a glassy amorphous phase and thus provides an unusual example of direct transformation from crystal to glass. On the other hand, a slow dehydration promotes the formation of a polymorphic crystalline phase (alpha) of anhydrous trehalose. We also show how this duality crystalline state/glassy amorphous state has led, in previous studies, to a confused pattern of thermodynamic properties of the dehydrated state of trehalose dehydrate. Our results show, in particular, that the properties of the glassy amorphous trehalose obtained by dehydration are very close to that of the quenched liquid trehalose.