The electrochemical reaction mechanisms of FePx (x = 1, 2, 4) as electrode materials in Li batteries were analyzed by 57Fe Mossbauer spectroscopy, magnetic and NMR measurements. Depending on their stoichiometry, iron phosphides react with lithium along different pathways. FeP and FeP2 are fully or partially converted to a composite electrode of Li3P and Fe. For both, the same intermediate phases, namely FeP and LixFeP, were identified in cycling and FeP is partially regenerated on charge. Surprisingly, FeP4 reacts with Li through an insertion type process previously identified for earlier transition metal phosphides (Ti, V, and Mn), and no conversion reaction to Li3P/Fe is observed. The long-term stability of the composite electrode Li3P/Fe0 and of the intermediate phase LixFeP is analyzed.