Iodine is emitted from the ocean and helps to destroy ozone in the lower atmosphere before being taken up into aerosol particles. We measured the chemical forms of iodine in aerosols over the Atlantic Ocean, because some of these forms can return to the gas phase and destroy more ozone. Our results indicate that aerosol acidity exerts a strong control on iodine speciation and therefore on its recycling behaviour and impact on ozone concentrations.