<p>For the efficient production of high-purity lithium sulfide (Li₂S), a crucial raw material for sulfide-based solid electrolytes in all-solid-state batteries, the reduction of lithium sulfate (Li₂SO₄) with hydrogen sulfide (H₂S) gas was investigated. In this study, H₂S (<i>g</i>) was generated by dissolving iron sulfide (FeS) in a sulfuric acid (H₂SO₄) solution at 293&#xa0;K, and then reacted with Li₂SO₄ at 873–1073&#xa0;K. When the reduction of Li₂SO₄ with H₂S (<i>g</i>) was conducted at 1073&#xa0;K for 6&#xa0;h, high-purity Li₂S was produced. In these experiments, a deposit of excess elemental sulfur (S) was observed. The excess S was produced by the decomposition of H₂S (<i>g</i>) and the reaction between H₂S (<i>g</i>) and sulfur dioxide (SO₂) gas generated by the reaction of Li₂SO₄ with H₂S (<i>g</i>). In addition, the mechanism for the reduction of Li₂SO₄ with H₂S (<i>g</i>) was elucidated using a chemical potential diagram considering the chemical potentials of the species involved in the reaction. These findings demonstrate the feasibility of producing high-purity Li₂S by reducing Li₂SO₄ with H₂S (<i>g</i>) generated by dissolving FeS in a H₂SO₄ solution.</p> Graphical Abstract <p></p>