The binary phase diagram between the plastic crystals 2-methyl-2-nitro-1-propanol (MNP) and 2,2-dimethyl-1,3-propanediol (neopentylglycol, NPG) has been established by means of thermal analysis and X-ray powder diffraction from 273 K to the liquid state. The binary system is characterized by a wide eutectoid invariant corresponding to the solid- plastic transition and by a narrow plastic-liquid loop.