A complete 57Fe Mössbauer study was undertaken to elucidate the electrochemical mechanism occuring during the cycling of the FePx (x = 1,2)/Li batteries. Upon discharge, for FeP, the conversion reaction FeP + 3Li → Fe + Li3P was confirmed, whereas for FeP2, FeP and a disordered magnetic unknown phase A are observed. On charge, the starting material is partially reconstructed only in the case of FeP. Once the first cycle is achieved, the same redox process occurs in both the FeP/Li and FeP2/Li cells, between LiFeP, FeP, Fe° (or phase A) and Li3P.