We performed in-situ X-ray diffraction and ex-situ textural observations and thermodynamic calculations in order to confirm phase relations in the system MgO-H2O at high pressure and high temperature conditions. In-situ and ex-situ observations reveal that the brucite decomposition boundary has a negative slope from 11 GPa to 20 GPa. Phases obtained at 2.1 GPa and 1503 K were periclase and a fluid with a small MgO content while those at 12 GPa and 1473 K were periclase and an MgO-rich liquid. Quench textures suggest that brucite dehydrates at least up to 4 GPa but melts incongruently at higher pressure. The incongruent melting temperature of brucite, that is wet solidus temperature of periclase, decreases with increasing pressure up to 20 GPa. The experimental results are supported by thermodynamic calculations.