The stability of phase B, hydrous magnesium silicate (Mg23Si8O42H), has been investigated experimentally up to 20 GPa and 2500¿C. Phase B is synthesized by heating a powder mixture of forsterite and periclase under hydrous conditions at pressures above 15 GPa, and temperatures above 1500¿C, and is stable up to its incongruent melting temperature (2300¿C at 20 GPa), above which is decomposes into an assemblage of liquid and periclase. The remarkable stability of phase B indicates the possibility of its presence as a host for H2O in the deep upper mantle of the Earth.