Previous work shows that the response of bacterial bioreporters to aqueous Arsenic (As) is altered by solution chemistry. However, the influence of solid phase Fe precipitates on the detection of As by these engineered microorganisms is not understood. In this study, we applied bacterial bioreporters to assess the bioavailability of As(V) bound to the surface of a suite of Fe(III) (oxyhydr)oxide minerals. We found that As(V) bound to the surface of crystalline goethite (As:Fe = 1 mol%) was not detected by the bioreporters. By contrast, substantial fractions of As(V) (35–50%) adsorbed to nanocrystalline two-line ferrihydrite (As:Fe = 1–6 mol%) were detected by the bacteria, which suggests that structural properties of the solid phase play a key role in the detection of surface-bound As(V) by bioreporters. Our findings stress the importance of preventing the oxidation of Fe(II), which is common in As-contaminated aquifers, to ensure accurate As measurements when applying bioreporters in the field.