Uptake of BrONO2 onto submicron sulfuric acid aerosol was studied in a cylindrical flow reactor coupled to a chemical ionization mass spectrometer. On aerosol particles of ~0.1 μm radius, the reaction of BrONO2 with H2O to produce HOBr and HNO3 is very efficient, with the reaction probability &ggr;≈0.8 for H2SO4-content ≤70 wt%. The equilibrium constant for the gas phase process, BrONO2+H2O⇄HOBr+HNO3, was measured and the heats of formation of the bromine compounds were deduced. The conversion of HOBr back into BrONO2 via reaction with HNO3 can be neglected in the stratosphere. The heterogeneous hydrolysis of BrONO2, followed by photolysis of the HOBr product, is a major source of HOx in the lower stratosphere. The role of bromine as an activator of chlorine is also discussed. ¿ American Geophysical Union 1996