At least 93% of Fe(II) remained free, as defined by ferrozine assay under anoxic conditions in the presence of humic acid (HA) and two simulated landfill leachates of different maturities. However, tangential flow ultrafiltration showed a weaker but more extensive interaction of Fe with organic carbon (OC); 90% of Fe associated with the less mature leachate. Despite the existence of this weak interaction under anoxic conditions, there was no difference in iron(III) (hydr)oxide production whether HA was added prior to or coincident with the oxidation of Fe(II) on exposure to oxic conditions. Under oxic conditions ferrozine showed that more Fe(II) bound to OC, up to 50% to HA. However, this occurs via oxidation of Fe(II) to Fe(III), which is bound and then thermally reduced. This affinity for Fe(III) and the ability to carry out thermal reduction both increase with the maturity of the OC. The rate at which ferrozine-defined free Fe(II) was lost on exposure to dissolved oxygen was also enhanced by the more mature OC, while it was slowed by acetogenic leachate. The slowing must be a consequence of the filtration-defined Fe(II)/OC interaction. © 2012 American Chemical Society.