The partial vapor pressures over liquid or supercooled solutions of water and nitric acid and of water and sulfuric acid are calculated for temperatures below O¿ C. From these results the partial vapor pressures over the ternary system water-nitric acid-sulfuric acid (liquid or supercooled) have been estimated and compared with the available experimental data of Vandoni at O¿ C. Some possible implications for the formation of stratospheric aerosols are also discussed.