<p>Molten sulfide electrolysis has the potential to offer an energy-efficient and low-CO₂ technique for metal production and can be operated with a supporting electrolyte composed of barium sulfide and lanthanum sulfide. Barium sulfide is produced from barium sulfate, and lanthanum sulfide is produced from lanthanum oxide or lanthanum sulfate. Although various methods, such as carbothermic reduction, hydrogen reduction, and sulfidation using sulfur, have been investigated, an optimal method capable of producing barium sulfide for molten sulfide electrolysis is still sought after. In the current work, the carbosulfidation of barium sulfate in the presence of iron(II) sulfide was investigated. At 1473 K, where iron(II) sulfide is molten, barium sulfate was completely sulfidized through the formation of molten Ba–Fe–S phase. When lanthanum oxide or lanthanum sulfate was added to the reacting barium sulfate, both barium and lanthanum were sulfidized, confirming that electrolytes for molten sulfide electrolysis can be directly produced from sulfate and oxide raw materials.</p>