In 1990, Suenaga, Matsuo and Suga [1] published the heat capacities and phase transitions of neopentylglycol (2,2-dimethyl-1,3-propanediol) at low temperatures and pressures; here a solid-solid phase transition (s(3) --> s(2)) was observed at 60.1 K. In the present study, the phase behaviour of neopentylglycol was measured by high-pressure DTA in the temperature range 300-370 K at normal pressure and at pressures up to 700 MPa. The apparatus and measuring method have been described elsewhere [2-4]. The neopentylglycol was obtained from Aldrich (purity 99%) and used without further purification. Fig. 1 shows three typical thermograms of neopentylglycol at approx. 440, 640 and 666 MPa as obtained in the present investigations. Fig. 2 shows the temperature (T)-pressure (p) phase diagram that results from the measurements. The transition temperature for s(2) --> s(1) was found to be 315 K at normal pressure. As the apparatus is not suitable for low temperature measurements, the transition temperature for s(3) --> s(2) could not be determined at normal pressure. The s(2) phase is pressure-limited. It is destabilized at a triple point where the three solid phases, s(1), s(2) and s(3), coexist (363 K, 646 MPa); the triple-point data were obtained from extrapolations of the experimental T(p) curves by third-degree polynomials. The investigations are continuing [5].