Fe—Mn—B ternary system up to a 50% at. boron content was investigated. In the 850-1150 °C range and for B amounts from 33,3 to 50% at., there exists a large two-phase field, where the terms of two continuos solid solutions (Mn, Fe)₂B and (Mn, Fe) B are in equilibrium. For lower B contents a cubic iron manganese boride appears, namely the $\tau$ phase, whose boundary compositions are (Mn_0,78, Fe_0,22)₂₃B₆ and (Mn_0,40, Fe_0,60)₂₃B₆ respectively. In the high Mn field the $\tau$ phase is in equilibrium with Mn-B system the $\delta$ phase and at the opposite limit with $\gamma$-(Mn,Fe) and the (Fe_0,78, Mn_0,22)₂B term. The $\tau$-phase being stable up to 920 °C, at higher temperatures the $\delta$ phase is in equilibrium with the $\beta$-(Mn,Fe) solid solution.