Drop calorimetry was modified in order to adapt it to the measurement of the enthalpy of fusion of materials with above-ambient fusion temperatures, T-F, such as salt hydrates suitable as phase change materials. In the modified form, differential drop calorimetry, the energy delivered to the molten sample at temperatures above T-F and by the crystalline sample at temperatures below T-F are extrapolated to T-F to give the enthalpy of freezing. The calorimetric system was calibrated electrically and with acetamide (T-F = 353.3 K) and applied to the congruently melting magnesium nitrate hexahydrate, cadmium nitrate tertrahydrate, ammonium alum, and several hydrated salt mixtures. (C) 2003 Elsevier Science Ltd. All rights reserved.