The excess internal energy of one polymorph of a material over another may be determined from their heat of solution in a given solvent. (This energy is not to be seen as reversible heat of transformation of the polymorphs.) Thus the difference between the internal energy of amorphous and crystalline forms of sucrose, glucose and glucose monohydrate have been determined from measurement of their heat of solution in water. These differences are 21.2, 15.5 and 28.1 kJ mol(-1), respectively. This difference in energy is caused by differences in van der Waals interaction energy, the extent of and the total energy associated with H-bonding in the two solids, and their vibrational frequencies. The implication of these studies and further use of the procedure are discussed in general terms, and it is proposed that this method is more accurate than the usual method of integrating heat capacity-temperature data. The method can be used for determining the excess energy at 0 K of materials which decompose or melt incongruently, and when neither the heat capacity of the high temperature phase nor the heat of phase transformation can be measured.