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In this study, the kinetics and stable isotopic fractionation of nitrate (NO 3 −) reduction in Fe(II)/Fe(III) homogeneous/heterogeneous systems were investigated at circumneutral pH (i.e., pH 6.5 to 7.5) and ambient temperature (24 °C). Of the suite of iron (oxy)(hydr)oxides examined, NO 3 − reduction by Fe(II) was only significant in the presence of layered double hydroxide green rust (GR) minerals. Comparison of type 1 and type 2 GRs demonstrated that type 1 (GR(Cl−)) had the fastest NO 3 − reducing ability, with an Fe(II)-normalized pseudo first-order kinetic rate constant of k obs (NO 3 −) = 16.2 × 101 M−1 d−1. Anion concentrations (SO 4 2− or Cl−) and the ratio of [Fe(II)] to [Fe(III)] also influenced reduction kinetics. Ammonium (NH 4 +) was the predominant reaction end-product (>50%) with the remaining reduced N species concluded to be comprised of N 2(g) as other intermediates (nitrite (NO 2 −), nitric oxide (NO) and nitrous oxide (N 2 O) were not detected during NO 3 − reduction. Apparent nitrogen (δ15N-NO 3 −) isotopic fractionation (15ε) for GR(Cl−) and GR(SO 4 2−) were were calculated to be 12.9 (CI: 8.2, 16)‰ and 37.9 (CI: 34, 41)‰, respectively. Similarly, oxygen (δ18O-NO 3 −) isotopic fractionation (18ε) differed between GR(Cl−) and GR(SO 4 2−): 4.53 (CI: 2.5, 5.8)‰ and 14.4 (CI: 12, 16)‰. However, the ratio of 18ε:15ε for both minerals could be fitted to a linear regression of slope 0.369 (CI: 0.361, 0.377). These findings reaffirm the importance of GR minerals to NO 3 − reduction, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers. These results also demonstrate that N and O stable isotope kinetic fractionation analyses as well as the ratio of 18ε:15ε can provide a means to distinguish between this abiotic anoxic reaction from anaerobic bacterial NO 3 − reduction processes. [ABSTRACT FROM AUTHOR]