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^*D^1/2 (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, HD^1/2 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.