The application of high pressure DTA to the study of solid-solid and liquid-solid transitions is critically assessed. Pressure-temperature (p-T) phase diagrams of mercury chalcogenides (HgS, HgSe and HgTe), for pressures up to 2GPa, are presented and discussed. The pressure influence on the zinc blende-cinnabar transition is elucidated. A phase transition mechanism is proposed that explains the origin of the hysteresis observed in DTA experiments.