The study of chemical interactions between Mo and B4C layers in multilayer structures depending on their total thickness (d) and thickness ratio (Γ) was carried out using X-ray photoelectron spectroscopy (XPS) method. The results showed significant interactions of materials within the multilayer structures. Specifically, in the range of d from 1.8 to 3.4 nm and Γ value between 0.24 and 0.46, a complete conversion of B4C into MoBxCy was observed, while retaining some amount of molybdenum. The insertion of tungsten barrier layer to the Mo-on-B4C interface led only to insignificant reduction of component MoBxCy. Based on the data obtained by XPS, theoretical models were developed and applied to X-ray reflectometry data during fitting procedure. The application of XPS has proven to be highly effective in constructing models and obtaining numerical values when fitting reflection curves.