Molybdenum is a nutrient important for a variety of biological functions, most notably nitrogen fixation. Molybdenum is also the most concentrated trace metal in seawater, in part owing to its stability and weak adsorption behavior. However, molybdenum availability is limited through sorption reactions, particularly in environments rich in sulfide minerals. In fact, Mo scavenging in the oceans occurs primarily in anoxic basins where Mo is sorbed by sulfide minerals including pyrite. This fact has been used extensively in the paleooceanographic work as a proxy for sediments deposited under strongly reducing conditions. This work examined the sorption of two major molybdenum species, molybdate (MoO42-) and tetrathiomolybdate (MoS42-), on synthetic pyrite (FeS2) as a function of solution composition. Both MoO42- and MoS42- partitioned strongly on FeS2 under a range of conditions and ionic strengths. Molybdate and tetrathiomolybdate adsorption obeyed a Langmuir isotherm. Although both MoO42- and MoS42- adsorbed most strongly under moderately acidic conditions, MoO42- sorption was reversible while MoS42- adsorption was irreversible even at high pH. X-ray absorption spectroscopy (XAS) revealed that molybdate formed bidentate, mononuclear complexes on FeS2, while tetrathiomolybdate formed Fe-Mo-S cubane clusters on pyrite . The high affinity of MoS42- for FeS2, as well as its resistance to desorption, supports the hypothesis that thiomolybdate species are the reactive Mo constituents in reduced sediments and may control Mo enrichment in anoxic marine environments. Consequently, Mo enrichment would occur only in environments where the sulfide activity is sufficiently high to cause molybdate to convert to MoS42-, consistent with the observed trends in the oceanographic record.