This study identifies the electrochemical and solution chemical controls on the production of Fe(II,III) (hydr)oxides formed by the electrolysis of Fe(0) metal, also knows as Fe(0) electrocoagulation. EXAFS spectroscopy and X-ray diffraction were used to characterize the solids produced as a function of: i) applied current, which corresponded to iron(II) production rates of 30-300 mu M min(-1), ii) pH and iii) background electrolyte. Two systems were investigated where: i) the dissolved oxygen (O-2) concentration was maintained at 0.1, 0.3 and 3.0 mg L-1 and ii) the O-2 drifted in response to varied Fe(II) addition rates. A narrow range of 0 2 separated the domains for Fe(II,III) and Fe(III) (hydr)oxide formation. At O-2( )>= 0.3 mg L-1, Fe(III) solids dominated, while Fe(II,III) (hydr)oxides were the principal phases at O-2 = 0.1 mg L-1. The highest fraction of Fe(II,III) (hydr)oxides formed in the O-2 drift experiments at the highest Fe(II) production rate, i.e. 300 mu M min(-1). The background electrolyte determined the type of Fe(II,III) (hydr)oxide that formed: NaCI solutions favored magnetite and NaHCO3 solutions favored carbonate green rust. Our results are consistent with an Fe(II,III) (hydr)oxide formation pathway where Fe(II) addition after O-2 depletion leads to rapid (<10 min) transformation of precursory Fe(III) precipitates. (C) 2018 Elsevier Ltd. All rights reserved.