An innovative approach coupling DFT calculations and simple thermodynamics is developed for determining surface energies and morphologies of Co(Ni)MoS nanocatalysts used in hydrotreatment. According to sulfo-reductive conditions, three regimes are put forward. For high chemical potentials of sulfur, the promoter localization is found to be stable on both particle edges. Intermediate sulfiding conditions enhance the selective affinity of Co for the S edge only, whereas Ni remains stable on both edges. Finally, the destabilization of the mixed phases is found in highly reductive regimes.