Changes in the composition of embedded pyrite (FeS2) particle electrodes in 1 M LiClO4 propylene carbonate solutions as a function of the applied potential have been examined in situ by S K-edge fluorescence X-ray absorption near edge structure (XANES), using a specially designed cell that minimizes attenuation of low energy X-rays. Pyrite electrodes that had been scanned from 3.0 V versus Li/Li+, i.e. close to the open circuit voltage, down to 1.0 V (fully discharged state, i.e. 4e−-reduction) and then half recharged (2e−-reoxidation) by scanning the potential in the positive direction up to 2.2 V versus Li/Li+, revealed features consistent with the presence of Li2FeS2, in agreement with in situ Fe K-edge results reported earlier by this research group. Moreover, only subtle changes were discerned between the in situ S K-edge XANES of the half-, and fully-recharged electrodes. This close resemblance may reflect similarities between the spectral signatures of Li2FeS2 and Fe1−x S (pyrrhotite), which is the main product of the discharge reaction. Evidence for the formation of elemental sulfur and Li2S, which are believed to be minor products of the reaction, was obtained from analysis of the first differential S XANES and selected difference spectra. The compositional variations of the embedded pyrite particles throughout the course of the electrochemical processes occur in the presence of a persistent sulfate coating.