Behavior of iodine fission product is of prime importance for short-term radiological consequences in a severe accident occurring on a pressurized water nuclear reactor. Iodine speciation in the reactor coolant system is commonly predicted with severe accident simulation software devoted to the transport and deposition of fission products and structural materials, for instance, the SOPHAEROS module of ASTEC. In these calculation tools, chemical equilibrium is assumed to be reached instantaneously whatever the conditions are. However, some thermodynamic data are still uncertain because of lack of experimental data. Quantum-chemical calculations can be appropriate tools to estimate equilibrium constants in a first step and maybe later to determine some kinetic constants for further implementation in such codes to better assess iodine chemical behavior. This paper is an attempt to calculate some equilibrium reactions for relevant reactions that are susceptible to impact iodine chemistry. The accuracy obtained for such calculations depends on the basis set used. Moreover, relativistic effect has to be taken into account for heavy atoms like iodine or cesium to get reliable predictions.