In case of a severe nuclear accident on a Pressurized Water Reactor, radioactive fission products can be released in the environment and represent a hazard for the human. In order to better predict the progress of the accident and the release of fission products with the use of dedicated simulation tools, the knowledge of all physicochemical phenomena is necessary. This research is focused on the re-vaporisation, up to 750°C, of iodine-containing fission products’ deposits, particularly CsI and AgI, from the surface of the primary circuit, composed of partially oxidized 304L, 316L steels and Inconel 600 alloy. The results have revealed a strong influence of the re-vaporisation atmosphere composition on the quantities and the chemical species nature of re-vaporized caesium and iodine. In steam atmosphere, iodine and caesium are released integrally from the surface, mainly as CsI. In presence of air, iodine is released integrally, mainly as I2(g), whereas caesium is partially retained on the surface of the steel as caesium chromate, resulting from the interaction between caesium and chromium (III) oxide present in the oxide layer. The use of an online optical spectroscopic technique such as IBB-CEAS allowed to follow the I2(g) re-vaporisation kinetics. The latter exhibits two release peaks, suggesting several mechanisms leading to I2(g) formation. A similar result has been observed during the re-vaporisation of AgI in presence of air. Finally, thermodynamic equilibrium computations have been performed in order to identify the main reaction pathways leading to the formation of gaseous molecular iodine in presence of air.