RECENT measurements of inorganic chlorine gases’ and hydrocarbons’ indicate the presence of reactive chlorine in the remote marine boundary layer; reactions involving chlorine and bromine can affect the concentrations of ozone, hydrocarbons and cloud condensation nuclei, The known formation mechanisms of reactive halogens require significant concentrations of nitrogen oxides(3-5), which are not present in the unpolluted air of the remote marine boundary layer(6). Here we propose an autocatalytic mechanism for halogen release from sea-salt aerosol : gaseous HOBr is scavenged by the aerosol and converted to only slightly soluble BrCl and Br-2, which are released into the gas phase, Depending on the sea-salt concentration and given a boundary layer that is stable for a few days, gaseous HOCl and HOBr may reach molar mixing ratios of up to 35 pmol mol(-1). We calculate that HOBr and HOCl are responsible for 20% and 40%, respectively, of the sulphur (IV) oxidation(7,8) that occurs in the aerosol phase, The additional S(IV) oxidation reduces the formation of cloud-condensation nuclei, and hence the feedback between greenhouse warming, oceanic DMS emission and cloud albedo, We also calculate significant bromine-catalysed ozone loss.