The uptake and reaction of HOI and IONO₂ with dry and frozen NaCl/NaBr salt surfaces was investigated using a coated-wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOI and IONO₂ react with both surfaces to form the di-halogens IBr and ICl, which are released into the gas phase. On mixed Cl⁻/Br⁻ surfaces in which the concentration of Cl⁻ greatly exceeds that of Br⁻, both HOI and IONO₂ react to form ICl or IBr. ICl reacts further with Br⁻ to form the observed gas-phase product, IBr. Formation and release of ICl is only observed once the Br⁻ concentration has been chemically depleted. The sum of IBr + ICl in the gas phase was found to remain constant during the reaction, and to account for all of the HOI/IONO₂ taken up, indicating no loss of iodine to the surface, and a catalytic role of iodine in the activation and release of bromine and chlorine from mixed salt surfaces. The uptake coefficient of HOI on a frozen, mixed salt surface of similar composition to sea-water at 243 K was >10⁻². Similar results (γ>10⁻²) were obtained for the uptake of HOI onto dry, mixed salt surfaces of similar composition at 298 and 243 K. Lower limits for the accommodation coefficient of HOI on frozen salt (243 K), dry salt (253 K) and H₂SO₄ (253 K) of 0.05, 0.12 and 0.3, respectively were obtained. The implications of these results for tropospheric halogen chemistry are briefly discussed.