The electrochemical reaction of lithium with crystalline intermetallic eta'-Cu6Sn5 has been studied by means of in situ X-ray diffraction. The reaction of Li with Cu6Sn5 proceeds in two distinct steps. A first discharge plateau at approximately 0.4 V vs. Li is attributed to the coexistence of Cu6Sn5 and a ternary Li2CuSn-type phase. Upon further reaction Li2CuSn coexists with a Li-rich tin phase (Li4.4Sn) and metallic Cu at a potential below 0.1 V. During the charge, the initial intermetallic is reformed in a multistep reaction. First, some Li can be removed from Li4.4Sn making Li4.4-xSn with the same structure. As Li is removed from Li4.4-xSn it reacts with the available Cu to make Li2CuSn. Then, lithium can be removed from Li2CuSn, making vacancies in the structure to form Li2-xCuSn. When x in Li2-xCuSn reaches about 1, the removal of further lithium results in the formation of a second phase, Cu6Sn5. The 0.8 V plateau during the charge of Li/Cu6Sn5 cells is associated with this coexistence of Li2-xCuSn and Cu6Sn5 The in situ experiments show that both the Li4.4Sn and the Li2CuSn phases can exhibit a range of Li stoichiometries, especially as Li is removed from their structures. When Li/Cu6Sn5 cells are charged and discharged above the lower plateau, to prevent the continual expulsion and reincorporation of Cu from the cathode grains, better cycling is obtained than when cycling includes the lower plateau where metallic copper is formed.