Li-ion battery anodes fabricated from crystalline MoO3 nanoparticles display an anomalous reversible capacity of 630 mAhg(-1) when cycled at high rate. The nanoparticle anodes show no degradation in capacity for 150 cycles, where micron sized MoO3 particles are shown to fail after several cycles. Theoretical calculations elucidate the Li-ion intercalation mechanism and explain the reversible capacity.