Experimentally reversed quartz solubilities at 250°C and at 250, 500 and 1000 bars yield values of the logarithm of the molality of aqueous silica of -2.126, -2.087 and -2.038, respectively. Extrapolation of quartz solubility to the saturation pressure of water at 250°C results in a log molality of aqueous silica of-2.168. These solubility determinations and analyses of fluid pressures in geothermal systems indicate that pressure is significant when calculating quartz equilibrium temperatures from silica concentrations in waters of deep thermal reservoirs. The results of this investigation, combined with other reported quartz solubility measurements, yielded a pressure-sensitive "silica geothermometer" for fluids that have undergone adiabatic steam loss of t°C = 874 - 0.156P/(log m Si(OH) 4 · 2H 2O ) 2 + 411 log m Si(OH4 · 2H 2O + 51 (log m Si(OH) 4 · 2H 2O ) 2 where P is the fluid pressure in bars and mSi( OH) 4 · 2H 2O represents the molality of aqueous silica measured in surface samples. The geothermometer is applicable to solutions in equilibrium with quartz from 180°C to 340°C and fluid pressures from H 2O saturation to 500 bars.