Nonquasibinary sections [0.5CuInSe₂∙0.5In₂Se₃]–MnIn₂Se₄, [0.5CuInSe₂∙0.5MnSe]–MnIn₂Se₄ and [0.5CuInSe₂∙0.5In₂Se₃]–[0.5CuInSe₂∙0.5MnSe of the quasiternary system CuInSe₂–МnSe–In₂Se₃ were investigated  by using DTA, X-ray and MSA. Phase diagrams of these sections were built. The obtained data were used to construct  the isothermal section of the quasiternary system CuInSe₂–MnSe–In₂Se₃, at determining  ternary eutectic points in the subtriangles, directions arrangements of monovariant curves and boundaries of crystallization phases in the subsolidus.