A characterization has been carried out on the dihydrate and anhydrous crystalline form of trehalose (alpha-D-Glucopyranosyl-alpha-D-glucopyranoside) which is considered anomalous in respect to other disaccharides in conferring to organisms the ability to be dried at high temperatures and then to rehydrate without damaging cellular functions. The thermal dehydration of the dihydrate trehalose, T-h, has been studied with the aim to rationalize the stability and the kinetically controlled transformations of T-h into the anhydrous forms. This has been reached by carrying out the dehydration process at different scan rates and by preparing the several polymorphic species. A description is therefore presented of the water-trehalose interaction which controls the thermal stability (or metastability) of the several forms obtained under controlled water depletion. Enthalpy changes and temperatures of transition are also reported and compared with literature data.