The interaction between the redox chemistry of Fe and Cu at nanomolar has been studied in UV-treated seawater. The oxidation of Fe(II) was studied as a function of concentrations of Cu(II) and Cu(I) from 0 to 200 nM. The effect of added H2O2 (0–500 nM), pH (6.0–8.5) and NaHCO3 (2–9 mM) on the Fe(II) rate constants was studied at Cu(II) levels (0–200 nM). To understand the competition between Fe and Cu, the reduction of Cu(II) to Cu(I) was also studied as a function of oxygen (air-saturated and anoxic seawater), Fe(II) (0–200 nM) and H2O2 (0–300 nM). The Fe(II) oxidation was accelerated by the presence of Cu(II) and Cu(I). This acceleration has been explained by the redox coupling between Fe and Cu, competition for different inorganic species (hydroxyl and carbonate groups studied independently) and by the formation of Fe-Cu particles (cupric or cuprous ferrite). Superoxide played a key role in the oxidation rate of Fe(II) in the presence of Cu(II). The presence of Fe(II) caused a greater reduction of Cu(II) to Cu(I). This is directly related to the levels of oxygen, Fe(II) and H2O2 concentrations. The presence of Fe(II) produced a rapid formation of Cu(I) in the first 2–3 min of reaction. The Cu(I) is oxidized reaching a steady-state around 20 nM levels of Cu(I). These experimental results demonstrated that the presence of Fe and Cu strongly affected the inorganic redox chemistry of both metals in UV-treated seawater.