In situ measurements of chlorine oxide (CIO) obtained on 31 March 1991 with a new balloon-borne instrument are compared to results from a photochemical model which incorporates hydrolysis of N2O5 on sulfate aerosols. With the addition of this process, there is better agreement between calculation and measurement over most of the profile, except below 20 km where observed CIO is greater by as much as a factor of four. In a model which is constrained to reproduce the observed CIO below 20 km, ozone loss by catalytic cycles involving halogen oxides becomes larger than that from NOx, which would dominate under gas-phase or standard heterogeneous conditions. ¿ American Geophysical Union 1993