The reaction SO3+H2O+M has been investigated with a tubular flow reactor at a pressure of 85 Torr. Sulfuric acid has been identified as the most probable product of the reaction. Both, reactant SO3 as well as the product H2SO4 have been measured quantitatively using chemical ionization mass spectrometry. An upper limit for the reaction rate coefficient of 2.4¿10-15 cm3 s-1 has been inferred from the measurements. Implications for sulfuric acid and aerosol formation in the plumes of jet aircraft are discussed. ¿ American Geophysical Union 1993