Radiochemical methods are quite suitable for studying the behaviour of radioiodine under the dilute conditions relevant to nuclear reactor accidents. Species selective adsorbents are able to distinguish between various inorganic and organic gas-phase iodine species. A solvent extraction procedure for determining aqueous phase organic iodide, free iodine, I– and IO
fractions has been investigated and found to be valuable, although large inaccuracies in the separation of I– and IO
can occur for solutions of pH above 10. The extraction of potentially-volatile species in the aqueous phase gives a measure of iodine species volatility consistent with observed values of the partition coefficient. Indirect measurement suggest that the partition coefficient of HOI at room temperature exceeds 30,000.