The isothermal section of the Mn-Sn-Zn system at 500°C was determined with 20 alloys. The alloys were prepared by melting the pure elements in evacuated quartz capsules. The alloy samples were examined by means of X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. A new ternary phase Mn 4 Zn 8 Sn (k) was found to have a bcc structure with a lattice parameter a = 0.92508 (5) nm. Its composition range spans 25 to 35 at. pct Mn, 4 to 8 at. pct Sn, and 55 to 70 at. pct Zn. The Zn is substituted for Mn in Mn 3 Sn, Mn 2 Sn, and Mn 3 Sn 2 . The solubility of Zn in Mn 3 Sn, Mn 2 Sn, and Mn 3 Sn 2 was measured to be about 17, 12, and 4 at. pct, respectively. The phase boundaries of the liquid and b-Mn phases were well established. The following 3 three-phase equilibria were well determined: (1) b-Mn + e-MnZn 3 + Mn 3 Sn, (2) k + Mn 3 Sn + Mn 2 Sn, and (3) L + k + Mn 2 Sn. The additional 5 three-phase equilibria, which are e-MnZn 3 + k + Mn 3 Sn, e 1 -MnZn 3 + e-MnZn 3 + k, e 1 -MnZn 3 + k + L, Mn 2 Sn + L + MnSn 2 , and Mn 3 Sn 2 + MnSn 2 + Mn 2 Sn, were deduced and shown with dashed lines in the present isothermal section.