Phase relation for Mg2SiO4+20.4wt.%H2O composition was studied in the pressure range from 11 GPa to 22.5 GPa. The sequence of the phase transitions in the temperature range from 800¿C to 1000¿C with increasing pressure was as follows; Forsterite + Water → Phase A + Clinoenstatite+ Water → Phase E+ Water → Superhydrous phase B + Phase F+ Water → Brueite Phase F (+Water). Coexistence of brucite and phase F was observed at pressures above 20 GPa. These phases are stable at least up to 22.5 GPa and around 1000¿C, and are the candidates for the hydrous phase in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle. ¿ American Geophysical Union 1995