The differential thermal analysis (DTA) technique has been applied in a multi-anvil high-pressure apparatus to study the dehydration reaction of brucite (Mg(OH)2) to periclase (MgO) plus H2O at 4 to 6 GPa. At 4 GHa and 1030 ¿C, endothermic and exothermic peaks due to dehydration and rehydration were observed during heating and cooling cycles, respectively. These peaks were shifted to higher temperature with increasing pressure (1080 ¿C at 5 GPa; 1120 ¿C at 6 GPa), suggesting that the reaction has a positive dT/dP slope within this pressure range. The stability of brucite under deep mantle condition is discussed. ¿ American Geophysical Union 1991