The phase equilibria in the ternary systems Fe-B-N, Co-B-N and Ni-B-N have been established from X-ray powder diffraction data and isothermal sections at 900 degree C and 1 atm Ar have been constructed. In all three systems the binary transition metal borides form two-phase equilibria with hexagonal boronnitride. The iron-boron binary system has been reinvestigated by means of X-ray powder, metallographic and melting point analysis. 'FeB//2' and 'Fe//3B' are not stable in thermodynamic equilibrium and 'FeB//1//9' rather represents a solid solution phase of iron in beta -rh boron than a binary compound. A complete constitutional T-X diagram is presented.