The T(z) phase diagram of the system Mn3zGa2(1 − z)Se3 is determined from differential thermal analysis and X-ray diffraction measurements. Single-phase solid solutions with an adamantine structure are found to occur in the composition range 0 < z < 0.26 at temperatures below 660°C, but extended out to z = 0.57 above that temperature. Below 660°C, two separate ordered structures occur, corresponding to the monoclinic ordered structure of Ga2Se3 and to the tetragonal one of MnGa2Se4, while between these two there is a partially ordered zincblende form characterized by stacking faults, as previously observed for Ga2Se3. This phase extends above 660°C but at higher values of z changes to a disordered zincblende form.