The heterogeneous chemistry of HOI, ICl and IBr on sea salt and sea salt proxies has been studied at 274 K using two experimental approaches: a wetted wall flow tube coupled to an electron impact mass spectrometer (WWFT-MS) and an aerosol flow tube (AFT) coupled to a differential mobility analyser (DMA) and a chemical ionisation mass spectrometer (CIMS). Uptake of all three title molecules into bulk aqueous halide salt films was rapid and controlled by gas phase diffusion. Uptake of HOI gave rise to gas-phase ICl and IBr, with the latter being the predominant product whenever Br⁻ was present. Only partial release of IBr was observed due to high solubility of dihalogens in the film. ICl uptake gave the same yield of IBr as HOI uptake. Uptake of ICl on NaBr aerosol was accommodation limited with α = 0.018 ± 0.004 and gas phase IBr product has a yield of 0.6 ± 0.3. The results show that HOI can act as a catalyst for activation of bromine from sea-salt aerosols in the marine boundary layer, via the reactions: HOI_aq + Cl⁻_aq + H⁺_aq → ICl_aq + H₂O_l and ICl_aq + Br⁻_aq → IBr_aq + Cl⁻_aq.