A shared-cost action on Iodine Chemistry and Mitigation Mechanisms (ICHEMM) has been undertaken as part of the 5th Euratom Framework Programme on Nuclear Fission. Organisations from seven countries have been involved in an integrated programme of experiments, analysis and code development with the following objectives: To provide new experimental data for use in formulating models for volatile iodine destruction or transmutation reactions which are not routinely included in severe accident iodine chemistry codes; To investigate other possible mitigation mechanisms or accident management measures to favour the conversion of volatile iodine species to non-volatile forms under severe accident conditions; To provide new experimental data on iodine behaviour under conditions specific to BWR containments under accident conditions, and To quantify the effects of the identified mitigation mechanisms on the predicted iodine source term for representative accident sequences. New experimental measurements have been made of the rate of radiolytic decomposition of gaseous methyl iodide under a range of conditions (temperature, dose rate, humidity, atmosphere etc). A mechanistic model of the process has been developed to assist in interpreting the experiments and understanding the main reactions. An empirical model suitable for use in containment iodine chemistry codes has also been developed. Experiments to measure the rate of reaction of gaseous I2 with ozone in condensing conditions did not give definitive results due to problems with the analytical techniques that could not be surmounted. However, the limiting values obtained were consistent with measurements in non-condensing conditions. Rates of destruction of ozone at different surfaces were measured, showing that destruction is fastest at painted surfaces and at elevated temperatures. However, the reaction rate is not fast enough to significantly affect the kinetics of gaseous I2 radiolytic destruction. The effects of different chemical additives on the rate of destruction of irradiated aqueous CH3I were experimentally studied with the aim of generating data for use in further development of procedures for accident management to retain organic iodine as far as possible in the reactor containment sump under severe accident conditions. At low temperatures (=500C) and without additives, destruction by radiolysis is clearly dominant compared with hydrolysis. The pH effect does not appear significant with the range 5 � 9. Experiments with additives show that ammonium sulphide, sodium thiosulphate and especially Aliquat 336 and cysteamine are strong candidates for further investigations to ultimately improve accident management procedures. The reaction of I2 and CH3I with reactive metal surfaces typically found in BWR containments (Al, Cu, Zn) has been studied. The reaction of gaseous I2 with all three metals was fast; in the aqueous phase, significant absorption only occurs on Cu as the other two metals cannot form an insoluble iodide product. Methyl iodide deposition onto the surfaces was much less efficient. Five integral tests have been performed in the CAIMAN facility, extending the database on iodine volatility and organic iodide formation to higher temperature and dose rate, as well as condensing steam conditions. These tests showed that the predominant form of iodine in the gas phase was organic, mainly formed by reaction with dissolved organic materials in the sump. A State-of-the-Art report on iodine chemistry and mitigation mechanisms was produced at the start of the project. During the course of the work, new models have been developed for various processes and these have been incorporated into containment chemistry modelling codes. An assessment has also been made of the relative importance of the different mechanisms for mitigating iodine volatility. Plant calculations have been carried out to determine the effect of the new models on the iodine release in prototypic PWR and VVER sequences. This work has shown that the effect of the different mitigation mechanisms is dependent on the reactor type and sequence in question, with a greater impact being observed for the VVER sequence. This programme has produced an extensive body of new data which can be used to validate iodine chemistry models under a range of conditions of relevance to severe reactor accidents. Various new models have also been developed during the course of the work that can be used in plant studies to improve the reliability of source-term assessments.