Li3Fe(MoO4)(3) undergoes a complex electrochemical reaction with lithium in which the reduction of Fe3+ and Mo6+ takes place along the first discharge of the cell at about 2.4 and 1.8 V, respectively. The intercalation process involved is fully reversible for low lithium contents, Li3+xFe(MoO4)(3) with 0 < x < 1, the inserted compound Li3+1Fe2+(MoO4)(3) retaining the Li3Fe3+(MoO4)(3) parent framework with only a slight increase of the cell volume (0.85%). In contrast, the electrochemical reaction of Li3Fe(MoO4)(3) with five lithium ions originates an irreversible decomposition of this material into a mixture of a FeO-type compound and an amorphous lithium-molybdate phase. This in situ formed blend is electrochemically active, being able to intercalate and deintercalate three lithium ions at an average voltage of 2 V (reversible specific capacity of 150 Ah/kg). The full discharge of the cells (down to the vicinity of 0 V) proceeds through the complete and irreversible reduction of Li3Fe(MoO4)(3) with 25 lithium ions, resulting in the breakdown of any existing crystalline framework. (c) 2005 The Electrochemical Society. All rights reserved.