Thesis: 129I is chronically released from spent fuel reprocessing plants in the marine environment, e.g. Orano La Hague, and in the atmosphere, it can be re-emitted from the marine compartment to the land by gas or aerosol emission. The objective of this PhD is the development of a new analysis method of iodine 129 in order to evaluate the re-emission of iodine from the marine compartment to the land by gas or aerosol emission. For this purpose, two automatic samplers have been installed on the IRSN La Hague Technical Platform. Both are connected to a proportional counter to monitor 85Kr activities. 129I and 85Kr are simultaneously released by the Orano La Hague reprocessing plant, so it can be used as a tracer. Automatic samplers are composed of filters to trap aerosols, and charcoals to trap gaseous iodine. They were selected after various tests in order to be compatible with the analysis method. The difficulty for the iodine analysis is due to its extreme volatility and its multiple degrees of oxidation. The extraction of iodine from its matrix by acid digestion was developed and optimized through a design of experiment. This is followed by a purification by solid phase extraction, developed and adapted to a direct measurement by ICP-MS. The physico-chemical characteristics of iodine can induce non-spectral interferences such as memory or matrix effects. An optimal measurement medium has been then developed to avoid them. Moreover, as ICP-MS can present spectral interferences, the isobaric interference due to 129Xe+, present as an impurity in argon, has been eliminated thanks to a collision/reaction gas. The newly developed method allows the measurement of iodine 129 directly or after a chemical treatment. The detection limit was estimated to be 11 mBq L-1 for direct measurement and 2 mBq after chemical treatment, which is 10 times lower than current methods.