Kinetic investigations of the polymorphic form alpha of anhydrous trehalose have been performed below its apparent melting temperature (T-m) by differential scanning calorimetry (DSC) and X-ray diffraction. The results reveal a spontaneous isothermal vitrification process which indicates that the phase alpha is in a very unusual superheating situation. This behavior has been attributed to the fact that the effective melting temperature (T-m(eff)) of the phase alpha is likely to be located far below the glass transition temperature (T-g) of this compound. The high viscosity of the liquid trehalose between T-m(eff) and T-g is thus invoked to explain the long lifetime of the phase alpha in this temperature range.