Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature
range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray
diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph
I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed.
The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from
ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several
times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values
of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions
calculated for the two polymorphs.