An understanding of radioiodine volatility is important in relation to nuclear reactor safety. In this paper, findings of an ongoing experimental assessment of radioiodine partitioning are described. Radiochemical methods of determining liquid and gas phase speciation are presented along with experimental results demonstrating the validity of these methods. These results also indicate that the majority of the gaseous organic iodides detected at TMI-2 following the accident were in a low molecular weight form such as methyl iodide. It is concluded that, for 10–4 M CsI solutions, the partition coefficient increases by an order of magnitude with increasing pH and, the aqueous and gaseous concentrations of both molecular iodine and organic iodide decrease with increasing pH over the pH range of 4 to 12.