The description of containment iodine behavior in reactor accident sequences involves a combination of iodine volatility effects, deposition/revaporization processes, and mass transport of iodine species. The formation and retention of volatile organic iodides such as methyl iodide, CH3I, are important factors in determining iodine source terms. Tests have shown that the formation of organic iodide was enhanced by radiation and iodine in the chemical form of I2. Methyl iodide, the only organic iodide detected in measurable quantities, formed predominantly in aqueous solution even when the organic material was introduced to the gas phase.