Ab initio calculations on the modes of incorporation of water in quartz suggest that, apart from the incidental inclusion of molecular water as fluid inclusions, the most prevalent mode of uptake of water in α-quartz is structural and involves the replacement of Si by four protons in a process formally described as the [4H]si substitution. Existing ab initio computer calculations were used as a basis on which to determine the equilibrium concentration of [4H]si defects in quartz as a function of relevant temperatures and pressures. These calculations indicate that [4H]si concentrations are in the range 10−7-10−4/Si atom increasing rapidly with increasing water pressure at constant temperature, while above 300°C the defect concentration decreases monotonically with increasing temperature in all cases, implying that the defects must react on heating to produce molecular water. Ab initio results have also been used to establish the concentration of interstitial water molecules in quartz under the same conditions. In this case the concentrations are vanishingly small and of the order of 10−10/Si atom at 500°C and 1.0 GPa, implying that the diffusion coefficient for water on interstitial sites in quartz must also be extremely low.