Phase stability and phase transformation were studied in the Fe–Co–Sb ternary system for the three sections: CoSb–Fe 0.56 Sb 0.44 , 30at.% of Sb and 75at.% of Sb. In the first section, we find a continuous solid solution without any secondary phase. The unit cell volume increases as a function of X Fe /(X Fe +X Co ). At 30at.% of Sb, the B8 and the bcc-A 2 phases are obtained across the whole Fe–Co section. On the CoSb 3 side (75at.% of Sb), Fe atoms cannot completely substitute for Co atoms in the skutterudite structure. Below 5at.% of substitution of Fe for Co in CoSb 3 , only the D0 2 phase is present while for high concentration of Fe, marcasite and Sb phases coexist.