Liquid and supercritical H2O, the most abundant and important non-crystalline component in rock forming processes, is assumed to exhibit continuous PVT behaviour at high pressures and temperatures. However, a number of experimental observations at high pressure and temperatures challenges this concept. Direct measurements of PVT properties of H2O at supercritical conditions are very difficult. Information on the PVT behaviour at these P-T conditions may be gained by certain dehydration reactions. An experimental re-examination of the reaction Mg(OH)2 (brucite) = MgO (periclase) + H2O has been conducted between 0.4 to 3.5 GPa and 650 to 1200 °C. The study was performed with a piston cylinder apparatus using the in-situ differential pressure analysis (DPA)-technique. The data reveal that the P-T slope of the equilibrium boundary is not continuous but exhibit a fine-structure which is characterised by three inflections at 1.1 GPa/830 °C, 1.9GPa/945 °C and 2.7 GPa/1040 °C. Because the solid phases, Mg(OH)2 and MgO, exhibit no structural changes at these conditions, these effects are thought to be due to anomalous PVT behaviour of supercritical H2O.