The mechanism of a phase transition in electrochemical lithium insertion into alpha -MoO3 was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). alpha -MoO3 was deposited on a special Au micro grid for TEM observation by pulsed laser ablation. The phase transition ranged from x = 0 to 0.25 in LixMoO3 upon electrochemical lithium insertion into alpha -MoO3. In this two-phase coexistence region, the XRD (020) line of the lithium inserted phase (Li0.25MoO3) grew at the expense of the (020) line of the pristine alpha -MoO3 as lithium ions were inserted. However, the intensity of the lithium-inserted phase was much weaker than expected in the initial stage of the phase transition. TEM observation revealed that lithium ions were inserted randomly into some preferred crystallites of alpha -MoO3 to form a disordered phase in the initial stage, which explained the presence of the induction period upon the growth of the lithium inserted phase. (C) 2000 Elsevier Science B.V. All rights reserved.