In this study, we investigated the reaction mechanism of the FePS3 electrode in all-solid-state lithium secondary batteries that utilized sulfide-based solid electrolytes by X-ray diffraction patterns, X-ray absorption spectra, Raman spectra, and density-functional theory (DFT) calculations. In discharge-charge measurements, the reversible discharge-charge reaction (FePS3 + xLi(+) + xe(-) reversible arrow LixFePS3, 0 <= x <= 1.5) was confirmed. With this reaction, Li+-inserted FePS3 with low crystallinity was formed with the reduction of iron during the discharge cycle, and crystalline FePS3 appeared along with the oxidation of iron during the charge cycle. Raman spectra showed that P2S64- units were not destroyed during this discharge-charge cycle. In the second cycle, the discharge voltage of the batteries that used FePS3 increased relative to that at the first cycle. The reversible change in chemical states of iron and sulfur was confirmed by X-ray absorption. The first-principle calculation explained the experimental results of the change of crystalline phase and the increase in the discharge voltage. Further, the calculation results indicated that not only iron but also sulfur was oxidized and reduced from the first charge cycle onwards. (C) 2018 The Electrochemical Society.