The nature of the product of the electrochemical reduction of Li2FeS2 has been investigated. This product was completely reoxidized in intimate contact with a lithium ion-conducting solid electrolyte. The product is regarded as alpha -Fe, although usually, Fe is electrochemically inactive in contact with a solid electrolyte. In order to understand the reason for this discrepancy, the product was investigated by X-ray diffraction and Mossbauer spectroscopy. Its diffraction pattern did not show clear peaks and its Mossbauer spectrum was attributed to superparamagnetic Fe-0. These results suggest that the reduction product consists of non-crystalline, small particles of Fe dispersed in Li2S. Because of the small particle size, most of the Fe atoms are in contact with US. This environment is considered to permit the continuous reoxidation to Li2FeS2, resulting in its electrochemical activity.