Phase relations and phase stabilities have been derived for the ternary systems RE─B─N (RE = Nd, Sm, or Gd) at elevated temperatures (1400°C and above) by means of X‐ray powder analysis. Under the experimental conditions selected, various ternary compounds are found to be stable: Nd3B2N4with the Ce3B2N4type and (Nd,Sm,Gd)BN2with the PrBN2type. Phase equilibria at 1400°C and under 105Pa of argon are mainly characterized by the incompatibility of the RE metals Nd, Sm, and Gd with BN due to the competing equilibria between the RE tetraborides and the RE mononitrides. Each of the ternary compounds, however, is found to be in a two‐phase equilibrium withhex‐BN. Because of the different thermodynamic stabilities within the various structure series of ternary rare‐earth boron nitrides RE3B2N4and REBN2, the compound Nd3B2N4is observed only at temperatures below 1800°C and under 105Pa of Ar, whereas GdBN2is found to be stable only at temperatures above 1400°C under a partial pressure