The phase diagram of the binary system [2,2-dimethyl-1,3-propanediol] x (1) / [2,2-dimethyl-1,3- diaminopropane] 1-x (2) was studied by X-ray diffraction and DTA/DSC, for (2) also by 1H-NMR. The system is miscible over the whole concentration range 0 ≤ x ≤ 1 in the liquid state and in the plastic solid state, phase I, just below the melting point. At lower temperatures the system is demixing, and at room temperature two plastic mixed crystals coexist. The plastic phases of (1), (2), and (l) x (2) 1-x crystallize face centered cubic, Fm3m, Z = 4, the lattice constants decreasing linearely with increasing x, and the lattice constants are: (1) a (327K) = 880.3 pm , (2) a(243K) = 905.6 pm. By single crystal X-ray diffraction the structure of the ordered phase II of (1) was refined at room temperature, monoclinic, P2 1 /n, Z = 4, a = 596.9 pm, b = 1090.2 pm, c = 1011.0 pm, β = 99.74°. The results are in good agreement with the literature. The phase transition temperatures (in Kelvin) are T 1→m = 399.2, TM m→1 = 399.7, T 11→1 = 316.2, T 1→11 = 308.2 for (1); = 300.2, = 301.7, T 11→1 = 228.7, T 1→n = 194.2 for (2). Strong hysteresis is observed for the transition T 1→11 in (2). In the mixed systems (1) x (2) 1-x , 0 < x < 1, the disordered phases do not order even by quenching to liquid nitrogen temperature. High resolution 1 H-NMR measurements are reported for phase I of (2) as a function of temperature. The “liquid” 1 H-NMR spectrum is present far below the thermodynamic phase transition temperature T 11-1 , overlapping the wide line unresolved powder spectrum of phase II.