The reactive uptake of HOBr onto aqueous solutions containing Cl- and Br- has been studied using a wetted-wall flow tube reactor. The uptake was found to be limited by gas phase transport, enabling the measurement of the diffusion coefficient of HOBr in He(319¿48 torr cm2 s-1) and N2(84¿7 torr cm2 s-1) at 274 K (errors are 2 &sgr;). The same experiments allowed a lower limit to the accommodation coefficient of α>1¿10-2 to be calculated. Both Br2 and BrCl products were observed in the gas phase, the relative yield of which was dependent on the ratio of Cl- to Br- in the aqueous phase. At <Cl->=1M, and <Br->=10-3M, >90% of HOBr taken up by the film was released into the gas phase as Br2. At low Br- concentrations, BrCl was the dominant product. The role of pH in the efficiency of bromine release from the aqueous phase was examined by carrying out HOBr-uptake experiments with 1MCl-, 10-3MBr-, and pH between 4 and 10. At a pH of less than 6.5 at least 90% of the HOBr taken up onto the aqueous Cl-/Br- solution was released into the gas phase as Br2. Bromine was not released following HOBr uptake onto non-acidified solution. These experiments confirm the efficient activation of brominepredicted by recent models of halogen chemistry in the marine boundary layer. ¿ 1999 American Geophysical Union