A basic nature of cubic Ag4Hf3S8for electrochemical Li insertion was examined with relation to cell properties and Rietveld analyses forex-situX-ray powder diffraction patterns observed at several discharge steps. The composition dependence of cell voltage curves observed in the closed type Li/lM-LiPF6in PC/Ag4Hf3S8cell showed a long plateau at around 2.2–2.0 V up to LixAg4Hf3S8(x ≤ 3). The cell voltage decreased with an increase of Li intercalation and final lithiated phase was assigned to cubic LixAgyHf3S8(x > 3, × + y = 7). The electrochemical lithiation of Ag4Hf3S8at low constant current densities proceeded topochemically. The structures of lithiated phases were solved without changing the space groupP4332 (No. 212) for Ag4Hf3S8host, indicating that Hf3S8framework structure was kept through the lithiation.