Abstract <p>The specificity of the local atomic surrounding, the physicochemical state, and the electronic structure of five tin monolayers on a thin buffer layer of silicon formed epitaxially and their transformation as a result of thermal annealing are studied in situ in the present work. Using radiation from three synchrotron sources high-resolution surface-sensitive experimental methods, X-ray absorption near edge structure spectroscopy and X-ray photoelectron spectroscopy, were used as well as computer modeling. The possibility of oxygen atoms diffusion from tin to a buffer layer of silicon during storage of structures in laboratory conditions is shown. It is shown that during the formation of Sn/Si epitaxial nanolayers, there are no interatomic interactions at the heterogeneous boundaries of the structure, up to the possible formation of a tin-silicon solid solution, and, as a consequence, noticeable distortions of the electronic spectrum. However, high-temperature ultrahigh-vacuum annealing causes a phase rearrangement of the surface layers of such a structure accompanied by the redistribution of oxygen atoms from tin atoms to silicon of the epitaxial buffer with the formation of a thin layer of SiO₂. Within the thin tin-silicon transition layer of the studied structures and along its boundaries complex bonds can be formed between tin, oxygen, and carbon atoms.</p>