An instrument was developed for performing simultaneous X-ray diffractometric and differential scanning calorimetric (XRD-DSC) measurement using a power compensation DSC (pc-DSC). The feasibility of the technique is demonstrated by examining the thermal dehydration measurement of zinc formate dihydrate (Zn(HCO2)(2). 2H(2)O). The DSC curve shows apparent double endothermic peaks during the dehydration process, while X-ray diffraction analysis indicated that the resulting anhydrous product, Zn(HCO2)(2), was crystalline. The simultaneous measurement reveals that the DSC curve arises from a combination of the endothermic dehydration peak and the exothermic crystallization peak which resulted from the progression of the anhydrous products.