A two-dimensional sulphate aerosol model is used to assess the impact of combined NOx and SOx emissions from future High Speed Civil Transports on stratospheric aerosols and ozone. The model predicts the SOx emitted by this fleet of supersonics may double the aerosol surface area and the number of optically active particles below 20 km in the northern lower stratosphere. When the heterogeneous conversion of N2O5 to HNO3 on sulphate aerosols is taken into account, the predicted ozone changes due to future HSCTs emissions are smaller than those calculated when SOx and the subsequent increase in aerosol loading are neglected. It is worth noting that the doubling of the aerosol surface area may lead not only to a reduction in predicted ozone sensitivity to NOx, but also to an enhancement in ozone sensitivity to chlorine in the lower stratosphere. ¿ American Geophysical Union 1993