A transition from oxidizing to reducing conditions has long been implicated in increasing aqueous As in environments. However, it remains unclear whether reduction of arsenate, As(V), or As-bearing iron (hydr)oxides is the major mechanism of As release to pore-waters. Recently, however, reductive transformations of Fe (hydr)oxides have been shown to promote rather than limit As retention. Here we examine As desorption from ferrihydrite-coated sands pre-sorbed with As(III) at circumneutral pH under Fe-reducing conditions. Columns were inoculated with Shewanella putrefaciens strain CN-32, an organism capable of both As(V) and Fe(III) reduction. We reveal that upon the initial transition to reducing conditions, reductive transformation of As-bearing ferrihydrite results in arsenic(III) retention. However, over time there is a shift from reductive transformation to reductive dissolution of the As-bearing ferrihydrite coupled with prolonged release of As to the aqueous phase. Thus, arsenic retention may increase or decrease depending upon the initial degree of surface coverage and duration of reducing conditions. In the short term, immediately following a transition to anaerobic conditions there is potential for increased As retention on newly formed ferric/ferrous (hydr)oxide phases; however, prolonged reduction will result in both the dissolution of ferric (hydr)oxides and desorption of arsenic.