This work addresses the study of the dynamics of the excited states of CO in shock heated Mars’s atmosphere. The model accounts for thermal non-equilibrium between the translational energy mode of the gas and the vibrational energy mode of individual molecules. Furthermore, electronic states of molecules are treated as separate species, allowing for non-Boltzmann distributions of their populations. Two approaches have been considered and compared: i) state-to-state and ii) quasi-steady state. Both strategies are applied to the modeling of the flow behind a normal shock wave, for Mars entry conditions. The results indicate that the use of a quasi-steady state approach can lead to a substantial dissociation of the excited CO molecules, thus leading to a drastic underestimation of the radiation heat flux.