WATER plays an important role in geodynamic processes in the Earth's upper mantle: for example, hydrogen (as water) affects the amount and composition of magma generated by partial melting1 and a trace amount of hydrogen (∼0.001 wt% water) can markedly weaken the dominant upper-mantle mineral, olivine2,3. Migration of hydrogen ions may be responsible for the anomalously high electrical conductivity of the asthenosphere4. The quantitative importance of hydrogen in mantle processes must depend on how much water or hydrogen can be stored in nominally anhydrous olivine, and on where hydrogen resides in the olivine lattice. Here we report the results of hydrothermal experiments on olivine single crystals, which show that at 1,573 K and 50-300 MPa, olivine can accommodate as much as 0.0034 wt % water. Hydrogen solubility depends on hydrogen fugacity and oxygen fugacity to the first and the one-half powers, respectively, indicating that hydrogen ions are associated with either oxygen interstitials or magnesium vacancies. Extrapolation of our data to a depth of ∼ 100 km under oceanic areas yields a substantial hydrogen solubility (0.03 wt % water), demonstrating that olivine may indeed be a primary sink for hydrogen in the upper mantle.