Bacteriogenic iron oxides bacteria and poorly ordered iron oxides, such as ferrihydrite. Given their high surface reactivity and surface area, they have been shown to be efficient sorbents of  aqueous contaminants. The present study investigates the redox stability of naturally occurring BIOS and the fate of their sorb contaminants (As and Sr). Results indicate that BIOS samples (composed of ferrihydrite and smaller amounts of lepidocrocite and goethite)  from a wetland area and gold mine tailings undergo rapid microbial reduction in the presence of a well known iron-reducing bacterium (i.e., Shewanella putrefaciensCN32). In fact, the reduction rates observed for the various BIOS samples far exceed those of synthetic iron oxides (ferrihydrite). The results also show that the presence of sorbed Sr and As (present as outer-sphere and inner-sphere complexes, respectively) stabilizes BIOS during microbial reduction by blocking reactive sites onto the iron oxides.  Finally, the fate of Sr and As during reduction mirrors that of Fe(II), suggesting that all sorbed contaminants are likely sorbed onto the iron oxides.