The second work package will be dedicated to the study of the atmospheric D/H ratio. We will investigate the coupling of the isotopic fractionation effects occurring in the bulk atmosphere (CEFE and PHIFE) and the impact on the abundance of H and D atoms in the high atmosphere. Furthermore, the diurnal cycle of the D/H ratio resulting from the regolith-atmosphere interaction will be studied. Finally, the subsurface transport model will be integrated into the GEM-Mars GCM to investigate the influence of the regolith and all isotopic fractionation effects on the seasonal cycle of the atmospheric D/H ratio.