Phase equilibria were established in the ternary system Mn–Y–Zr for an isothermal section at 800 °C by use of X-ray powder diffraction, light optical microscopy and quantitative EPMA. No ternary compounds were observed. Mutual solid solubilities among the binary phases were found to be generally less than ~ 2 at.%. Solubility of Mn, Y in αZr at 800 °C was 2.1 at.% Mn and 2.9 at.% Y, whereas no Mn dissolves in α Y. A reinvestigation of the binary solid solubility limits of Mn 2 Zr 1− x at 800 °C by quantitative EPMA and X-ray powder full profile analyses revealed a smaller homogeneous region, Mn 2 Zr 1− x , 0 ≤ x ≤ 0.20, than previously reported in literature. There is little solubility of Y in stoichiometric Mn 2 Zr and maximum solubility of Mn, Zr in αY was determined from as cast alloys to be 1.8 at.% Mn and 2.4 at.% Zr. Based on this experimental findings and the literature data relevant to the binary systems a thermodynamic calculation of the ternary system was attempted, requesting a significantly higher heat of formation of Mn 2 Zr than previously reported. A nearly pseudobinary section was experimentally established for the join Y–Mn 2 Zr with a maximum eutectic at 1072 ± 10°C at 57 at.% Y (calculated at 1064 °C and 58 % Y). Three ternary eutectics were defined: L⇔Mn 2 Zr + Mn 2 Y + Mn 23 Y 6 at 1090 ± 10 °C at a composition of Mn 75 Y 23 Zr 2 (calculated at 1087 °C and at Mn 72 Y 26 Zr 2 ), L ⇔ (αY) + Mn 2 Y + Mn 2 Zr at 870 ± 10 °C at a composition of Mn 40 Y 57 Zr 3 (calculated at 883 ° C and at Mn 36 Y 63 Zr 1 ) and L ⇔ (αY)+ (βZr) + Mn 2 Zr at 1054 ± 10 °C at a composition of Mn 26 Y 15 Zr 59 (calculated at Mn 24 Y 21 Zr 55 and at 992 °C).