The phase equilibrium glass formation in the AsSe–AsMn2 system have been studied by the methods of physico-chemical analysis (differential thermal analysis, X-ray powder diffraction, microstructure examinations, microhardness measurements, and density determinations) and the state diagram AsSe–AsMn2 system was constructed. The AsSe–AsMn2 cut is a partially quasi-binary section of the As–Mn–Se ternary system. In a system at room temperature solid solutions extend to 1.5 mol % AsMn2, on the basis of AsSe – to 2 mol %.At slow cooling a region of glass formation on the basis of AsSe reaches 13 mol % AsMn2. While at quenching in liquid nitrogen region on the based AsSe glass formation extends to 30 mol. % AsMn2.