The subject of this study is the formation of the phase composition and structure in nanoscaled CoSbx (30 nm) (1.82 6 x 6 4.16) films deposited by molecular-beam epitaxy on substrates of oxidized monocrystalline silicon at 200˚C and the following thermal treatment in vacuum from 300–700˚C. It is established that after deposition, the films are polycrystalline without texture. With increased Sb content, the formation of the phase composition in the films takes place in such a sequence as is provided by the phase diagram for the bulk state of the Co–Sb system. With annealing in vacuum at temperatures above 450–500˚C, sublimation occurs not only for the crystalline Sb phase, but for the antimonides as well. This is reflected in the phase composition change by the following chemical reactions: CoSb2 600˚C → Sb↑ = CoSb, CoSb3 600˚C −→ Sb↑= CoSb2, CoSb3+ Sb↑ 600˚C → CoSb3 and leads to increases in the amounts of the CoSb and CoSb2 phases and decreases in the amounts of CoSb3. CoSbx(30 nm) (1.826x64.16) films are found to be thermostable up to ≈350˚C.