Black shales and euxinic marine sediments contain elevated Mo concentrations which may convey useful information about paleoredox conditions at deposition sites. Among users of this information are environmental managers, who have become interested in reconstructing oxygen-stress histories for modern coastal environments. Optimal use of Mo profiles necessitates a better understanding of Mo capture mechanisms in euxinic sediments. A key control on Mo behavior is the activity of molecular H₂S (aq), which must reach ~10^-5 M before the geochemically inert MoO₄^2- in seawater will begin to convert to particle-reactive thiomolybdates (MoO_xS_4-x^2-, x=0-3). This conversion is catalyzed by mineral surfaces, possibly explaining why Mo scavenging from anoxic marine waters occurs more often within sediment pore waters, rather than in overlying waters. When H₂S is below the critical threshold, Fe monosulfides are able to scavenge Mo weakly, but this form of Mo is not geologically robust; scavenging is reversed by even brief O₂ penetration episodes. Pyrite is relatively refractory during O₂ penetration episodes and appears to be the most important host-phase for Mo in both euxinic sediments and black shales. Mo is captured irreversibly by pyrite surfaces as Fe-Mo-S cluster complexes. These complexes may survive for >10⁸ y in black shales. The reaction pathway leading to these products requires Mo^VI reduction, which is accomplished by S^o-donors (mainly polysulfides) through an unusual, ligand-induced electron transfer process. Polysulfides not only convert Mo to forms suitable for scavenging by pyrite, but promote pyrite growth as well. This proposed mechanism of Mo scavenging by pyrite can operate over a broad redox range. No critical pE threshold for Mo scavenging can be specified other than that conditions sufficient for biological sulfate reduction must exist.