We present approach adapted to study the interface (composition and extension) of the X-ray multilayer mirror using angular resolved photoelectron spectroscopy (ARXPS). In the approach we rely on the concept of average effective attenuation length L (EAL) of the photoelectron and not on the inelastic mean free path λ_i (IMPF) that allows us to take into account the contribution of elastically scattered electrons and to increase the accuracy of the determined thickness of layers. We apply the developed approach to study the formation of interfaces in the multilayer periodic Mo/Si mirror. Chemical composition and significance of the interfaces depending on the number of periods were investigated by means of ARXPS spectra decomposition technique. Formation of molybdenum silicide MoSi2 at the interfaces was revealed. It was shown that molybdenum silicide with different thickness is formed at the interfaces depending on the films order. In addition, it was established that increasing of periods number of the [Mo/Si] system leads to a decrease of the interface extension.