[en] In case of a severe accident in a nuclear power plant, the behaviour of iodine is one of the major issues because of its radiotoxicity. Special attention is paid to the gaseous species as a result of their mobility and so the ease with which they can escape into the environment. During the Phebus-FP tests, a significant fraction of gaseous iodine was observed in the containment that is not well accounted by the simulations. Possible explanations are kinetic limitations or the presence of caesium sinks such as molybdenum in the reactor coolant system that would limit the formation of stable condensed forms mainly CsI. In order to get additional experimental data, a test facility (GAEC test facility) was developed with the aim to study the chemical reactions between iodine, caesium and another element in conditions representative of the primary circuit. This paper presents the first results obtained with GAEC test facility during the vaporization of CsI and MoO₃ under steam. It was clearly observed that molybdenum promotes the fraction of gaseous iodine at 150 C. degrees (temperature of the cold leg). Molecular iodine I₂ was shown to be the predominant iodine gaseous species and caesium poly-molybdates under condensed form were identified by Raman microspectrometry. This last result consolidates the idea that molybdenum acts as a caesium sink. A simple approach is proposed using Gemini2 code and a specific database to reproduce the formation of the caesium molybdates. Simulations performed with the new kinetic model implemented in the SOPHAEROS code gives promising results concerning the fraction of gaseous iodine. (authors)