[en] The paper reports PSI participation in the ICHEMM Project (Iodine Chemistry and Mitigation Mechanisms) of the EU 5th Framework Programme, which has the objective of identifying new possible mitigation processes, or accident management measures, favouring the conversion of volatile iodine species, especially organic iodine released during a severe accident at a Nuclear Power Plant (NPP), into nonvolatile forms. Significant effort has been expended, firstly to develop analytical methods for determining concentrations of the common iodine species (iodide, iodate, molecular iodine and methyl iodide), and secondly to provide missing and important information on decomposition phenomena and involved reactions. Experiments have been conducted at PSI to obtain data on thermal (hydrolysis) and radiolytic decomposition of CH₃I in aqueous solution under conditions as close as possible to those in an anticipated accident. Radioactive-labelled methyl iodide was used to monitor and quantify the decomposition and to determine the overall mass balance. Results have shown that CH₃I hydrolysis at 90^oC will dominate over radiolysis at the dose rates (max. 0.4 Gy.s^-1) employed in the experiments. Much increased decomposition rates of CH₃I have been achieved with chemical reagents (additives), such as sodium thiosulphate or ammonium sulphide. After completion of the ICHEMM Project, further advances have been made from experiments carried out on additives (especially in combination with Aliquat 336) in terms of a fast and complete retention of iodine. The data from these experiments are of direct relevance to severe accident management for both PWR and BWR systems. The impact of the results on possible strategies for management of radioactive waste has also been investigated. (author)