IO and BrO radicals are intermediates in the atmospheric photo-oxidation of iodo- and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss. ¿ 1999 American Geophysical Union