Iron (hydr)oxides such as ferrihydrite exert a dominant influence on arsenic retention within soils and sediments. Under anaerobic conditions, the reductive transformation of ferrihydrite, or other ferric (hydr)oxides, may release arsenic to the aqueous phase. However, reductive biomineralization may limit the release of arsenic, and promote arsenic sequestration, through its incorporation in (or on) secondary solids. Rather than promoting mobilization, dissimilatory reduction of Fe(III) within ferrihydrite by Shewanella putrefaciens diminished arsenic desorption. Various secondary sinks may retain arsenic, but we observe a strong correlation between arsenic retention and dissolved Fe(II), suggesting the formation of an Fe(II)-As(III) precipitate. In order to decipher the mechanism by which arsenic is retained upon ferrihydrite reductive dissolution/transformation, we examined a biologically produced precipitate using a host of spectroscopic and microscopic techniques. Precipitates formed at varying concentrations of Fe(II) and As(III) and multiple pH values were used to define conditions conducive to the formation of the solid phase—a potential ‘reduced' sink of arsenic. Synthetic precipitates resulting from the reaction of Fe(II) and As(III) have a nearly one to one stoichiometric elemental ratio. X-ray absorption spectroscopic (XAS) analysis of synthetic Fe(II)-As(III) suggests, on the basis of local coordination environments, that the Fe(II)-As(III) solid consists of brucite [Mg(OH)₂] like Fe(OH)₂ sheets with As(III) in double-corner and edge-sharing coordination to Fe octahedra. X-ray diffraction spectra, obtained both from conventional and synchrotron X-ray sources, further supports the formation of a sheet-like precipitate. Morphological and lattice features of the Fe(II)-As(III) precipitate, imaged using transmission and scanning electron microscopy, confirm a sheet structure of aged Fe(II)-As(III) solids. Contrary to the current paradigm of arsenic release under anaerobic conditions, elevated concentrations of dissolved Fe(II), typically formed through bioreduction of Fe(III), may stabilize arsenic relative to transport. The formation of an Fe(II)-As(III) phase would necessitate high levels of Fe(II) through intense Fe(III) reduction. Intense Fe(III) reduction requires the presence of a highly bioavailable Fe (hydr)oxide such as ferrihydrite, high organic carbon content, circumneutral pH, and warm temperatures (near 25 °C). Thus, soils in tropical regions undergoing seasonal redox cycling (flooding and drying) would be likely candidates for internal cycling of arsenic within oxidized and reduced solids.