The potential impact of high-speed civil transport (HSCT) aircraft emissions on stratospheric ozone and the sensitivity of these results to changes in aerosol loading are examined with a two-dimensional model. With aerosols fixed at background levels, calculated ozone changes due to HSCT aircraft emissions range from negligible up to 4--6% depletions in column ozone at northern high latitudes. The magnitude of the ozone change depends mainly on the NOx increase due to aircraft emissions, which depends on fleet size, cruise altitude, and engine design. The partitioning of the odd nitrogen species in the lower stratosphere among NO, NO2, N2O5, and HNO3 is strongly dependent on the concentration of sulfuric acid aerosol particles, and thus the sensitivity of O3 to NOx emissions changes when the stratospheric aerosol loading changes. Aerosol concentrations 4 times greater than background levels have not been unusual in the last 2 decades. Our model results show that a factor of 4 increase in aerosol loading would significantly reduce the calculated ozone depletion due to HSCT emissions. Because of the natural variability of stratospheric aerosols, the possible impact of HSCT emissions on ozone must be viewed as a range of possible results. ¿ American Geophysical Union 1993