Iodine is a fission product of major importance in a severe reactor accident, because volatile species exist under reactor containment conditions. Radiolytic oxidation of iodide ions is an important source of volatile iodine species. The SISYPHE tests provide an experimental database of prime importance for the study of the mass transfer between the sump and the atmosphere of a containment building under natural convection and in an evaporating flow regime. This phenomenon impacts greatly on the airborne iodine concentrations. The two main effects of evaporating conditions are to increase the kinetics of transfer and to change the steady state iodine concentrations. The well-known two-film model has been modified to extend it to this type of conditions. The agreement between the experimental results and modelling is satisfactory. However, when applied to typical reactor conditions, the impact of this improved modelling on gaseous iodine concentration is not as strong as other phenomena, for example remaining uncertainties concerning organic iodide production mechanisms. Correlations enabling the calculation of individual mass transfer coefficients for the liquid and the gas phases are proposed. The values resulting from these correlations agree well with those obtained from the test interpretations.