D-mannitol is a sugar alcohol with a melting temperature of approximately 167 degrees C. It has been identified as a phase change material for storing heat at a temperature range of about 150-180 degrees C. The outcome of the published research on its applicability for this purpose is inconsistent and sometimes contradictory. We identified that there is a need for further study on the material to overcome such uncertainty. Thus we conducted a series of thermal measurements to quantify its properties and suitability for a phase change thermal storage. Our differential scanning calorimetric measurements showed that the material has a melting temperature of 167 degrees C and fusion heat of 297 kJ/kg. However, our accelerated thermal cycling test revealed that this value decreases logarithmically to 249 and 240 kJ/kg after 100 and 200 cycles respectively. The material shows a significant level of sub-cooling in DSC measurement with a solidification temperature of approximately 110-120 degrees C which can render the material unsuitable for thermal storage purposes. However, our secondary measurement with a large quantity of D-mannitol shows that such level of sub-cooling is unlikely to occur in a macro-scale sample. We built and tested a thermal storage system containing 3.8 kg of D-mannitol to investigate its performance in larger quantities and evaluate the heat transfer properties of the heat exchanger mechanism in the storage system. We show that the material releases significant heat when it is cooled down to 150 degrees C which means implies the occurrence of solidification at a temperature above 150 degrees C.