Phase relations in Cr 3 S 4 and the substituted system Cr 3 S 4– x Se x are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected equilibrium phase crystallizing in the monoclinic space group I 2/ m , some samples exhibit a defect phase with Cr 2 S 3 ‐like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase‐pure Cr 3 S 4 . The defect phase can, however, be introduced in phase‐pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr 3 S 4 with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm −1  K −1 for the substituted system in comparison to the pristine material 2.3 Wm −1  K −1 .