Differential scanning calorimetry was used to investigate the thermal behaviour of 1,3-propanediol and its aqueous solutions. The melting temperature of pure 1,3-propanediol and its melting heat were determined as 249 K and 11400 J mol(-1) respectively. The 1,3-propanediol-water phase diagram presented three types of behaviour on heating (following annealing), depending on composition : high alcohol-content mixtures that exhibited a eutectic and dissolution (equilibrium phases), intermediate mixtures that vitrified uniquely and high water-content solutions that showed a residual glass transition and dissolution but no eutectic. The fact that crystallization of the eutectic was not observed for high water-content solutions was discussed in terms of relatively strong interactions between water and alcohol in the Liquid state, thus impeding crystallization of the alcohol. Computation of the congealing curve of water and the solubility curve of 1,3-propanediol, using the model of associated solutions, confirmed this hypothesis. Finally, a comparison between 1,3-propanediol and its isomer, 1,2-propanediol; was made; it was also discussed as to why the former crystallizes but not the latter.