Abstract The phase relations of the Fe–Ta–S system at 1223 K have been examined by sealed silica tube methods and its isothermal section of the phase diagram has been constructed tentatively on the basis of X-ray results. The novel ternary sulfide (Fe,Ta) 5S 8 (iron-rich intercalation phase Fe 0.25(Ta 0.5Fe 0.5)S 2) was found to exist in the iron-rich upper portion of the phase diagram. It crystallizes in the trigonal structures with space group P 3 ̄ m1 and the lattice constants of a=3.3656(9) Å, c=5.8140(11) Å. This compound exhibits a weak ferrimagnetic property with the Curie temperature T c of 600 K and a semiconducting behaviour with an electric resistivity of 0.135 Ω cm at room temperature. The isomorphous selenide (Fe,Ta) 5Se 8 gave similar results.