Spectroscopic and X-ray diffraction operando techniques were used to investigate polycrystalline antiperovskite (Li 2 Fe)SO as cathode materials in a Li-battery setup. During Li removal, several intermediate, relatively stable phases exist. At low charging, Fe is oxidized from +2 to +3, but at higher charging, S 2− is also partly oxidized to elemental sulfur, suggesting a cathode bifunctionality, and both redox processes seem reversible. On cycling (Li 2 Fe)SO in a battery, spectroscopy data suggest that a part of the Fe atoms irreversibly vacate the high-symmetry positions in the crystal lattice, in line with the broadening of X-ray diffraction peaks. Instead, new, relatively broad reflections appear in the X-ray patterns that might be explained by a crystallographic superstructure, corresponding to a doubling of the cubic unit cell axis, but the peak broadness indicates a lowering in crystallographic symmetry. Using a standard electrolyte and a moderate charging rate of C/10 results in typical capacity loss per cycle, but by using an electrolyte with low sulfur solubility, the (Li 2 Fe)SO cathode is stabilized, and charge densities of more than 200 mAh g −1 at a 1C charging rate are obtained. Additionally, a Li-deficient precursor (Li 0.8 Fe)SO served as a cathode material in a Na battery, providing presumably reversible Na intercalation and removal.