The solid-phase Se speciation after short-term (3 weeks) contact of selenite [Se(IV)] oxyanions with pyrite (FeS2) and troilite (FeS)wasinvestigated using X-ray absorption spectroscopy (XAS; X-ray absorption near-edge spectroscopy-extended X-ray absorption fine structure (XANES-EXAFS)). It was found that the nature of the sulfide mineral dictates the final speciation since respectively Se0 and FeSex were formed, meaning that the reaction mechanism is different and that these phases cannot be regarded as geochemically similar. The experimental results support the previously proposed sorption/ reduction mechanism for the reaction of selenite with pyrite (8). In the presence of troilite the reduction proceeds through the intermediate formation of Se0 by reduction of selenite with dissolved sulfide. XAS data recorded for the FeS2 and FeS were compared with different Se reference phases, ranging in oxidation state from —II to +IV, used for validation of the XAS analysis methodology. This methodology can in principle be used to analyze Se phases formed in “in situ” geochemical conditions such as high-level radioactive waste disposal facilities.