The thermal behaviour of both standard and low molecular weight heparins with anticoagulant properties has been studied by DSC after cooling to −100°C. During the rewarming, the endothermic effect that appears between −32 and −12°C is related to the molecular weight (MW) of the heparin fraction: the higher the MW, the higher the endotherm temperature. At high temperatures, the decomposition temperatures of the samples (indicated by an exothermic effect between 241 and 254°C) are in the same relationship as that obtained at low temperatures. The endothermic effect at low temperatures for heparins is similar to that exhibited in the DSC curves of other glycosaminoglycans such as hyaluronate solutions, heparin-dermatan sulphate mixtures or chondroitin sulphate solutions.