In the present paper, the formation of an interface region in the multilayer periodic Mo/Be structure with/without a B4C or Si barrier layer depending on the annealing conditions was studied using X-ray photoelectron spectroscopy. The formation of different beryllides at the interfaces Be-on-Mo and Mo-on-Be was explained by the impact of the deposition-induced exchange caused by ballistic collisions and surface free energy. The influence of the high temperatures on the thermal stability of Mo/Be multilayer systems without/with a barrier layer was studied. Since the appropriately selected barrier layers prevent the formation of the interlayer region of mirrors at room temperature, it was concluded that it would also lead to a weakening of interlayer diffusion in multilayer mirrors at higher temperatures. The effect of barrier layer insertion on the thermal stability of Mo/Be structures was analyzed in detail. It was established that regardless of the material, the introduction of a barrier layer: (i) limits the formation of beryllides with an increase in the annealing temperature at the Be-on-Mo interface; (ii) prevents the formation of MoBe2, while forming MoBe12 beryllide at the Mo-on-Be interface; and (iii) does not limit the beryllium oxidation process at the Mo-on-Be interface. This journal is