We performed experiments at 2.5, 6 and 9 GPa, and temperatures ranging from 1000 to 1400 °C in enstatite saturated conditions in theMgO–SiO2– H2O system using a multi-anvil apparatus, and determined the dependence of OH solubility in forsterite as a function of pressure and temperature. The abundance of OH in forsterite was determined using polarized Fourier transform infrared spectroscopy. The results show that OH solubility in forsterite increases with temperature only at 2.5 GPa. At 6 and 9 GPa, the OH solubility reaches a maximum at temperatures of 1175 to 1250 °C, depending on pressure, and then decreases at higher temperatures. Such behaviour is explained by the change of water activity in the fluid due to dissolution of silicate component. Using OH concentrationsmeasured on subsolidus samples we determined the thermodynamic parameters of OHincorporation in forsterite, such as change in internal energy and entropy.We find almost constantΔE of 37.1±6.7 kJ/mol andΔS of 82.8±6.8 J/mol/K. These parameters together with a ΔVsolids of 10.6 cm3/mol are used to calculate the fugacity of the real fluid (i.e. water+silicates) and compare it to the fugacity of pure water. Our results imply that estimates ofwater storage capacity atmantle depths greater than 80 km and above ~1250 °Cshould be substantially reduced compared to models that assume that water is a pure fluid.