A novel method was developed for determining the solubility of sodium nitrate in water solutions at pressures ranging from 248 to 300 bar and temperatures from 450 to 525 degrees C. Sodium nitrate solubilities determined by this method ranged from 293 mg kg(-1) at 248 bar and 525 degrees C to 1963 mg kg(-1) at 300 bar and 450 degrees C. A simple hydration mechanism was used to model the data. In addition, solubilities were determined for potassium and lithium nitrate salts to assess the effect of ion size on solubility. These experiments determined that the solubility was proportional to the ion size, so that solubility decreased in the order LiNO3 > NaNO3 > KNO3. These studies were motivated by applications for sodium nitrate in hydrothermal treatment methods, especially supercritical-water oxidation.