Reactions at the electrode/electrolyte interface of all-solid-state lithium batteries were studied for combinations of sulfide-based solid electrolytes with various Li4-xGe1-xPxS4 and Li-y-M (M = Sn, Si) alloys as the negative electrodes, using ac impedance, X-ray diffraction and energy-dispersive X-ray spectroscopy. The solid electrolyte at the interfacial region was found to decompose with the application of a current through the cells, resulting in the formation of a solid electrolyte interphase (SEI) layer. Resistivity changes at the interface varied depending on the electrolyte composition and the redox potential (vs. Li/Li+) of the negative electrode material. Lower resistances were observed with lower Ge contents in the solid electrolyte and the use of a Li-M alloy with a higher redox potential due to the formation of an electrochemically stable SEI layer during battery operation. In contrast, a combination of higher Ge content and an alloy with a lower redox potential led to a rapid increase in the SEI resistance and increase in its thickness. The presence of a Li-P-S compound with low ionic conductivity in the interfacial region was found to be related with the increase of interfacial resistance, leading to poor cycling characteristics. The formation of a suitable SEI layer is an important factor in the future development of all-solid-state batteries and this study serves to clarify the relationships between the formation of the SEI phase, the redox potential of the electrode and the sulfide-based solid electrolyte composition. (C) 2015 Elsevier B.V. All rights reserved.