Biologically mediated redox processes have been shown to affect the mobility of iron oxide-bound arsenic in reducing aquifers. This work investigates how dissimilatory sulfate reduction and secondary iron reduction affect sulfur, iron, and arsenic speciation. Incubation experiments were conducted with As(III/V)-bearing ferrihydrite in carbonate-buffered artificial groundwater enriched with lactate (10 mM) and sulfate (0.08-10 mM) and inoculated with Desulfovibrio vulgaris (ATCC 7757, formerly D. desulfuricans), which reduces sulfate but not iron or arsenic. Sulfidization of ferrihydrite led to formation of magnetite, elemental sulfur, and trace iron sulfides. Observed reaction rates imply that the majority of sulfide is recycled to sulfate, promoting microbial sulfate reduction in low-sulfate systems. Despite dramatic changes in Fe and S speciation, and minimal formation of Fe or As sulfides, most As remained in the solid phase. Arsenic was not solubilized in As(V)-loaded incubations, which experienced slow As reduction by sulfide, whereas As(III)-loaded incubations showed limited and transient As release associated with iron remineralization. This suggests that As(III) production is critical to As release under reducing conditions, with sulfate reduction alone unlikely to release As. These data also suggest that bacterial reduction of As(V) is necessary for As sequestration in sulfides, even where sulfate reduction is active.