Ni and Zn are important alloying elements in Mg-based structural materials and hydrogen storage alloys. The isothermal section of the Mg-Ni-Zn system at 400 degrees C was investigated by means of X-ray diffraction (XRD) analysis and electron probe microanalysis (EPMA). The NiZn3 phase was observed to be stable in the as-cast and annealed Ni25Zn75 and Ni23Zn77 alloys. The experimental results confirmed the existence of the ternary tau compound at 400 degrees C. NiZn3, Mg2Zn3, Mg2Ni, MgZn2, MgNi2, and tau show significant homogeneity ranges in the ternary system. The Mg2Zn11 phase is being stabilized in the ternary system at temperatures higher than the melting point of this compound in the Mg-Zn binary system. In the frameworks of the CALPHAD approach, the Ni-Zn system was re-optimized and the thermodynamic description for the Mg-Ni-Zn system was thermodynamically assessed for the first time. The obtained thermodynamic models reasonably reproduce the experimental data on the phase equilibria and thermodynamic properties of the phases. The solidification paths of the representative Mg-Ni-Zn alloys were predicted within the Scheil-Gulliver model using the thermodynamic assessment performed in the present work. The results of the Scheil-Gulliver modeling are in agreement with the EPMA results. (C) 2019 Elsevier B.V. All rights reserved.