Electronic and structural properties of materials generated by the reduction and subsequent oxidation of pyrite (FeS2) In a Li+-based solid polymer electrolyte (SPE) have been examined in situ by fluorescence Fe K-edge X-ray absorption fine structure (XAFS) in a Li/SPE/FeS2 battery environment. The in situ X-ray absorption near edge structure (XANES) of fully, i.e., four-electron, discharged FeS2 displayed features consistent with the presence of metallic iron, in agreement with information derived from in situ X-ray diffraction (XRD) and in situ Fe-57 Mossbauer effect spectroscopy (MES) reported by other groups. The two-electron reoxidation (half-recharge) of such fully reduced material yielded an in situ XANES very similar to that of genuine crystalline Li2FeS2 prepared and characterized in this laboratory. These spectroscopic findings support conclusions made purely on the basis of electrochemical information, which identified Li2FeS2 as an intermediate in the charge-discharge mechanism of FeS2 in room-temperature Li+-containing electrolytes.