Detailed electrochemical studies of the reaction of Li with combinatorially sputter-deposited BCC (x less than or equal to 0.4) and nanocrystalline (x greater than or equal to 0.4) Mo1-xSnx thin films are reported for 0.05 less than or equal to x less than or equal to 0.5. In situ X-ray diffraction (XRD) performed for x = 0.22, 0.36, 0.44, show no evidence for formation of new phases during the reaction with Li. During charge/discharge cycling, a definite and periodic variation of the BCC Mo1-xSnx (110) Bragg angle for x less than or equal to 0.40 is strong evidence for a topotactic Li insertion mechanism. Sensitive differential capacity (dq/dV vs. V) measurements on Li/Mo1-xSnx cells with x less than or equal to 0.36 show no new peaks for as many 75 cycles, supporting a topotactic insertion reaction. However, in the higher Sn content films (x greater than or equal to 0.4), the appearance and growth of new dq/dV peaks after the first cycle is consistent with the aggregation of Sn clusters and the formation of Li-Sn phases. Inclusion of about 20% oxygen in nanostructured films seems to prevent Sn aggregation and improve the cycling performance markedly. The best materials give specific capacities near 350 mAh/g, have densities near 9 g/cc and cycle well. This could be the first report of true topotactic Li insertion into a metal host. (C) 2004 The Electrochemical Society.