The solubility of silica glass in water has been determined through the temperature range 200 degrees C. to 420 degrees C. at a pressure of 300 atmospheres. Some previous investigations of solubility of silica in water, it is believed, have indicated a too great solubility as a result of failure to distinguish true solubility from colloidal dispersion. The solubility is essentially zero at temperatures below 200 degrees C. and increases to a maximum of 0.20 part silica per 100 parts water at 360 degrees C. Solubility falls off beyond 360 degrees C., but is only slightly less at the critical temperature of water. Solubility in the vapor phase of water appears to be a function of the density of the vapor phase, as well as of temperature. At high pressures, as the density of the vapor phase approaches that of the liquid, its ability to act as a solvent will approach that of the liquid.