Gaseous HCl and HBr react with sulfuric acid at rates that depend strongly on acid concentration over a range of acidities typical of stratospheric aerosols. We monitor the competition between immediate desorption and H→D exchange after HCl and HBr thermalize upon collision with the surface of deuterated sulfuric acid: the exchange probabilities decrease from 0.7 to 0.1 (HCl) and 0.9 to 0.2 (HBr) as the acid concentration is increased from 55 to 70 wt % D2SO4 at 213 K. These measurements imply that HCl and HBr desorb faster than they dissociate at higher acidities, impeding the formation of Cl- and Br- in more acidic aerosols. Residence time measurements of HCl molecules that do enter the acid, however, indicate that HCl is more soluble than expected at high sulfuric acid concentrations. ¿ 2001 American Geophysical Union