In case of severe accident in a water-cooled nuclear reactor, the amount of gaseous iodine released from the primary circuit is of particular concern because it can have a direct impact on the short-term iodine source term to the environment. As revealed by the Phebus FPT tests, assessment of the iodine forms at the break is a complex issue that requires to take into account not only the thermal and flow conditions in the circuit but also the behaviour of several elements such as caesium and molybdenum involved in iodine chemistry at high temperature. Moreover, reactions between these elements and with the carrier gas could be kinetically-limited due to the low concentrations and the short residence time of the fission products in the primary circuit coupled to the strong thermal gradients encountered in this part of the reactor. In order to reduce the uncertainties in the fraction of gaseous iodine, experimental and modelling studies have been launched in the framework of the international CHIP program. The work presented in this paper is part of this program and aims to investigate the effect of molybdenum using an open flow reactor where MoO3 and CsI vapours are injected under steam/argon atmosphere. In these conditions, the experiments clearly demonstrate that molybdenum increases the fraction of gaseous iodine released at low temperature and show that this effect is highly sensitive to the Mo/Cs ratio. Caesium polymolybdates identified in the solid state by Raman microspectroscopy are consistent with the species of the Cs2MoO4-MoO3 diagram suggesting that some part of caesium could be present in the gas phase as Cs2MoO4. Calculations based on thermodynamic equilibrium in the gaseous phase give the right trend but predict HI as the major species whereas iodine is recovered mainly under molecular form (I-2). Iodine speciation is better accounted with the new kinetic scheme developed for the {I, O, H} system and recently implemented in the ASTEC code.