Upper limits for the concentration of stratospheric gas phase SO3 have been inferred from a reanalysis of previous balloon-borne ion composition measurements made at altitudes between 29 and 41 km. Assuming steady state conditions for the production and loss of gaseous SO3 lower limits for the first-order loss rate coefficient of SO3 could be deduced. These lower limits are compared with calculated loss rate coefficients associated with potential loss processes for SO3 and implications for the formation of gaseous H2SO4 are discussed. In particular, the present analysis shows that no gas phase H2SO4 can be produced from the gas phase reaction of SO3 with H2O at altitudes above the stratospheric H2SO4/H2O aerosol layer.¿ 1997 American Geophysical Union