Concentrations of sedimentary molybdenum (Mo) have been used as a proxy for palaeoceanographic redox conditions based on the distinctive behaviour of Mo under oxic versus euxinic (i.e., anoxic and sulfidic) conditions. However, the mechanisms that govern Mo sequestration in various euxinic settings are not fully resolved. It has previously been proposed that sulfate-reducing bacteria (SRB), the main drivers and regulators of euxinic conditions, can actively take up and reduce Mo intracellularly and passively induce Fe-independent Mo complexation and reduction at their cell surfaces. However, uncertainties remain regarding the underlying interactions and relative contributions of these proposed biotic Mo sequestration pathways. In this study, systematic experiments were carried out to examine the interactions among Mo(VI) species (MoO42- or MoS42-), ferrous iron (Fe2+) and SRB with a focus on combinations of conditions that lead to reductive Mo precipitation. The speciation of aqueous Mo and composition, structure, oxidation states and bonding environment of precipitated Mo-sulfides were analysed using UV-vis spectrophotometry (UV-vis), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray absorption spectroscopy (XAS). Results indicate that SRB does not directly reduce Mo but, rather, plays a passive role in mediating Mo sequestration by providing sulfide and potential nucleation sites at their reactive cell surfaces for precipitation. However, even in the presence of SRB cells, Fe2+ was required for Mo precipitation in all conditions tested. By identifying the limiting (and non-limiting) factors in the Mo reduction and sequestration process, this study provides significant new insights for interpreting Mo palaeoredox proxies.