Differential scanning calorimetry (DSC) was used to investigate the thermal behaviour of 1,2-propanediol enantiomers and to study the R(-)1,2-propanediol-S(+)1,2-propanediol phase diagram. The melting temperature of the pure enantiomers was determined as 240 K and their melting heat as 8400 J mol(-1). The phase diagram exhibited two types of behaviour depending on composition : mixtures for which partial crystallization was observed and those in which no crystallization occurred. From the experimental results, a tentative equilibrium phase diagram is proposed. It is suggested that organic liquids for which crystallization is difficult or even impossible are mixtures of several molecular conformations. This is the case for racemic 1,2-propanediol. These liquids should be accounted for in phase diagrams.