The electrochemical reaction of lithium with the metal phosphide FeP2 is reported. Li uptake in this material was investigated using electrochemical studies combined with X-ray diffraction, X-ray absorption spectroscopy, and magnetic measurements. The uptake of six Li during the first discharge leads to a specific capacity of 1365 mAh/g, and an amorphous phase retaining substantial Fe-P bonding that displays superparamagnetic behavior and an extremely low transition temperature. Upon charge, 5.5 lithium can be extracted from this metastable ternary "Li-Fe-P'' phase, leading to a reversible capacity of 1250 mAh/g. The low (ca. 10-15%) irreversibility exhibited by FeP2 makes it an interesting candidate in the search for new negative electrode materials. (C) 2003 The Electrochemical Society.