Room temperature gas phase absorption cross sections for SO3 have been measured over the wavelength range 195 to 330 nm using a diode array spectrometer. The SO3 spectrum is continuous with weak diffuse vibrational band structure in the 225 to 295 nm region. Atmospheric photolysis rate calculations show that photolysis of SO3 in the 190 to 230 nm actinic window is important in determining the sulfur partitioning in the upper stratosphere. ¿ 1997 American Geophysical Union