The electrochemical reaction of lithium with eta'-Cu6Sn5, hereafter generally referred to as Cu6Sn5, has been investigated. Two complementary techniques, which provide a consistent picture of the structural changes that occur during discharge and charge, have been used in this study: in situ Sn-119 Mossbauer spectroscopy and X-ray diffraction (XRD). The data confirm earlier reports that Cu6Sn5 initially undergoes a phase transition on lithiation to form a Li2CuSn-type structure, and that further lithiation results in copper extrusion and, ultimately, the formation of a compound resembling Li4.4Sn. The Mossbauer and XRD data provide strong evidence for solid-solution behavior in the Li-Cu-Sn system. The reversibility of the reaction to regenerate the Cu6Sn5 structure is compromised by an inability of the extruded copper to be completely reincorporated during charge, particularly when Li/Cu6Sn5 cells are deeply discharged to 0 V vs. lithium metal, or if nickel rather than copper is used as the current collector. Part of the capacity loss observed on the initial cycle is attributed to irreversible reactions with tin oxides in the electrode and to reactions of the lithiated Cu6Sn5 electrode with the electrolyte.