Solid solutions films of mercury sulfide-selenide (HgS1–xSex) were synthesized on glass substrates by the chemical bath deposition method. Theoretical calculations of the boundary conditions for the HgS and HgSe formation in the mercury-tartrate-thiourea-selenosulfate system were made. The boundary conditions of HgS1–xSex were defined by the overlap area between the constructed HgS and HgSe formation zones. The X-ray diffraction and elemental analysis showed that the obtained films are single-phase and consist of HgS1–xSex substitutional solid solutions in zincblende modification. The effect of Na2SeSO3 concentration on the degree of S-Se substitution, as well as on the optical and morphological properties of HgS1–xSex films, was investigated.