The phase transformations of an Ag36.4Sb15.6Sn48 nanocomposite electrode, Ag3Sn/ Sn/SnSb, were investigated during electrochemical lithium insertion/extraction. The high performance gives 410 mAh/g for 300 cycles. Electrodes were prepared at selected voltage points in the discharge-charge curves and high-energy synchrotron X-ray data was collected in order to study the reaction mechanism in detail. Both Ag3Sn and SbSn insert Li stepwise to form compounds with the partial compositions Ag2-xLi1-xSn (0 less than or equal to x less than or equal to 1) and Li2+xSn1-xSb ( x less than or equal to 1). At lower voltages, Li7Sn2, Li22Sn5, AgLi, and Ag5Li8 could be observed. The intermetallic compounds before and after lithiation have a strong crystallographic relation. (C) 2004 The Electrochemical Society.