In the present work, formation of interfaces in the multilayer periodic Mo/Be mirror was studied using X-ray photoelectron spectroscopy. Chemical composition and significance of the interfaces depending on the number of periods were investigated by means of the XPS spectra decomposition technique. Formation of beryllide compounds at the interfaces was revealed. It was shown that two types of beryllide form at the interfaces depending on the film order: MoBeα, 4.0 < α < 5.0, at the Be-on-Mo (BOM) interface and MoBeβ, 1.0 < β < 1.6, at the Mo-on-Be (MOB) interface. The increase in the number of periods from 1 to 3 leads to suppression of the MoBeα formation at the BOM interface, while quality of the MOB interface remains unchanged. In order to rationalize the observed phenomena, an assumption on the interface formation mechanism was made. According to this hypothesis, the chemical composition asymmetry of the interfaces arises from the difference in the diffusion mechanism of Be atoms: surface diffusion prevails during the MOB interface formation, while bulk diffusion is favorable during the BOM interface formation. In this regard, suppression of the beryllide formation at the BOM interface with increasing number of periods indicates reduction of bulk defects in the Mo film.