The intermetallic Mg2Sn is a promising negative electrode material for rechargeable lithium cells. Preliminary cycling tests have demonstrated stable capacities at 400 mAh/g for 20 cycles. Magnesium stannide was produced by mechanically alloying magnesium and tin powders. Mechanical alloying can convert the equilibrium Mg2Sn phase to a metastable phase by the introduction of defects with extended milling times. In situ X-ray diffraction has shown that the cubic Li2MgSn phase, which is similar in size and structure to cubic Mg2Sn, is produced by lithium insertion into the equilibrium and metastable phases. The conversion from the metastable phase is irreversible, so subsequent lithium removal from Li2MgSn produces the equilibrium Mg2Sn phase. (C) 2003 The Electrochemical Society.