The effect of organic compounds on the production of volatile species of iodine was examined. The first step has been to identify candidates for detailed study through a series of scoping experiments, in which the chemical environment existing in CANDU reactor containment is simulated. These experiments have involved the irradiation of 10–5M CsI solutions labelled with131I containing a dilute concentration of a particular organic compound (8.5·10–3 to 1.4·10–1M). A total of 20 compounds and polymers have been tested to date. Results have shown that many of these compounds do enhance iodine volatility, and that the degree of volatility is related to system pH. Using the iodine partition coefficient (H=iodine concentration in the liquid phase/iodine concentration in the gas phase) as a basis, values as low as 300 have been observed for chloroform solutions. Conversely some compounds, such as phenol, have produced low volatility, with values of 1·105. As a reference, a partition coefficient of 104 has often been used in safety analysis.