The heterogeneous interactions of HBr and HOCl with cold sulfuric acid solutions have been measured with a low-temperature, coated-wall flow tube coupled to an electron-impact mass spectrometer. In particular, (i) the effective Henry’s law constants (H*) for the solubility of HBr in sulfuric acid solutions (40.3-60.5 wt %) have been determined by measuring the HBr partial pressure as a function of temperature, (ii) the values of HD1/2 of HOCl, where D is the liquid-phase diffusion coefficient, have been measured by monitoring the time-dependent uptake of HOCl over sulfuric acid solutions (59.7-69.3 wt %), and (iii) when the steady-state decay of gas-phase HOCl is monitored in the presence of an excess of HBr, the liquid-phase rate constant for reaction between dissolved HOCl and HBr has been measured to be 2 x 10(6) M(-1) s(-1) in 69.3 wt % acid at 228 K. When coupled with field observations from the Arctic boundary layer, these experimental results suggest that (i) heterogeneous interactions involving HOCl could be important in maintaining ozone loss rates by acting as an efficient activation mechanism for HBr in the springtime and (ii) as a result of the extremely large Henry’s law constants for HBr in dilute sulfuric acid, sulfate particles in the boundary layer may contain significant amounts of dissolved HBr.