A DSC study of the dehydration process of trehalose dihydrate (TREh) crystals has been carried out by tuning scanning rates of the product formation, under "controlled evaporation rates" (confined conditions). The rationale for this study is the construction of a dynamic diagram as a function of the time scale of trehalose transformations and the timescale of water effusion. Results at different rates of heating and of water evaporation indicate that only dehydration of TREh crystals and melting of the TRbeta form are constantly present. Amorphous state can be thermally produced under different ways: (a) by direct amorphisation of TREh; (b) by passing through the TREalpha form; (c) by cooling the liquid formed after TREbeta melting. The anhydrous TREalpha form is produced from TREh under gentle conditions of low scanning rates (large transformation times) and high evaporation rates (low humidity) even at relatively low temperatures (60 degreesC).