The electrochemical reaction of Cu3P with lithium occurs via a multistep process that leads to the formation of metallic copper, Li2CuP, and Li3P, as shown by electrochemical and X-ray diffraction measurements. The structural similarity between these three phases facilitates this topotactic "displacement'' reaction that is almost fully reversible on oxidation, re-forming crystalline Cu3P and expulsing Li. The system shows high reversibility during the first cycle, with an irreversibility of only 17%, good cyclability, and a reversible volumetric capacity of 2500 mAh/cm(3). These properties suggest it could be an interesting anode material for lithium batteries. (C) 2004 The Electrochemical Society.