The microstructure of Mo / Be multilayer periodic systems was studied by X-ray reflectivity and diffractometry and EXAFS spectroscopy. It was found that in the Mo / Be system, mixed zones of different compositions form at the boundaries. At the Mo-on-Be border, there is a mixed zone, similar in composition to MoBe22, and at the Be-on-Mo border, with MoBe2. As a result of thermal annealing for one hour, the structure of the transition boundaries in the multilayer system remains stable. With further annealing, diffusion processes occur that lead to the formation of another compound at the interface — MoBe2, instead of MoBe22, however, the period of the structure remains unchanged. This behavior explains the increase in the reflection coefficient of Mo / Be mirrors after annealing for one hour and a further decrease in the reflection coefficient with a longer annealing time.