Li3Fe(MoO4)(3) reversibly inserts 1 lithium ion per formula down to 2 V vs Li. A preliminary room temperature phase diagram for Li3+xFe(MoO4)(3) (0 < x < 1) is constructed combining electrochemical and in situ X-ray diffraction results. Single-phase regions are detected at x = 0, 0.75, and 1. The crystalline structure of the final compound Li3+1Fe(MoO4)(3) is derived from that of Li3Fe(MoO4)(3) by completely filling the tunnel formed by square pyramidal sites along the a-axis of the structure. The close structural relationship between the host and inserted compounds ensures a good capacity retention of this material over prolonged electrochemical cycling in lithium cells. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2139872] All rights reserved.