The I-atom sensitised decomposition of ozone in air at 1 atm pressure and ambient temperature has been investigated. Iodine atoms were produced by photolysis of 12 using visible light or of CH31 using ultraviolet light. In both cases, the quantum yield for 03 decomposition was 1.25 (-+0.11) per I atom. An important role is pxoposed for the self-reaction of IO radicals leading to higher oxides of iodine, IO + IO(+ M) ~ I20 2 (+ M) ~ higher oxides, which predominated over the bimolecular reaction leading to regeneration of I atoms, IO + IO ~ 2I + 02 , with k2a]k2b >/4. Simple computer modelling calculations indicate that reaction (2a) may be important in determining the fate of photolabile iodine species in the atmosphere. The consequences for the behaviour of radioiodine releases are also discussed.