The time-dependent uptakes of HBr and HCl by supercooled solutions of sulfuric acid have been studied using a low-temperature flow tube coupled to a mass spectrometer. From these experiments, typical values for the quantity H*D1/2 (where H* is the effective Henry's law constant and D is the liquid phase diffusion coefficient) have been measured to be 1100 and 3.3 (M/atm)(cm2/s)1/2 for HBr and HCl, respectively, on 59.6 wt % sulfuric acid at 210 K. by estimating values for the liquid phase diffusion coefficient, these measurements indicate that HBr is over 2 orders of magnitude more soluble in atmospheric sulfate aerosols than HCl. Similarly, HD1/2 for HOBr on 69.8 wt % sulfuric acid at 228 K has been measured to be 20 (M/atm)(cm2/s)1/2, implying that HOBr, too, is highly soluble in sulfuric acid solutions. By exposing sulfuric acid films simultaneously to HOBr and either HCl or HBr, the liquid phase reactions HOBr+HCl→BrCl+H2O and HOBr+HBr→Br2+H2O have been studied. The results indicate that the HOBr/HCl heterogeneous reaction may play an important role in the activation of stratospheric chlorine under volcanic aerosol conditions. ¿ American Geophysical Union 1995