The Se 3d XPS spectra showed evidence of reduction of Se(IV) by contact with the mackinawite surface when certain assumptions were made, but there was no clear evidence for reduction of Se(VI). A continuous oxidation of surface Fe(II) was observed for FeS in contact with Se(IV), as indicated by a decrease of intensities of Fe(II)-S XPS peaks. S 2p spectra showed some evidence of oxidation of surface S(-II) associated with Se reduction that resulted in formation of polysulfides. However, the apparent change in S 2p spectra could be the result of overlap between S 2p and Se 3p peaks. FeS particles became red and attached to the wall of reaction vessels within 3–4 days after contact with Se(IV), suggesting that a new solid-phase, possibly Se(0), may have been produced. The low signal-to-noise of the S 2p peaks observed after 15 and 30 days contact with Se indicates increased amounts of sulfur in solution. This supports the existence of a reaction of Se with FeS in which discrete precipitates are formed and sulfide is released to solution. These observations support the conclusion that Se(IV) sorbs onto the surface of FeS and oxidizes surface species of iron and sulfur. © 2011 American Institute of Chemical Engineers Environ Prog, 32: 84–93, 2013.