In oxic waters, dissolved Fe exists dominantly as Fe(III) complexes with strongly coordinating, siderophore-like ligands. In this study, we have determined an equilibrium Fe isotope fractionation of 0.6% (Δ 56Fe) between inorganic Fe(III) and Fe(III) siderophore complexes using the siderophore desferrioxamine B as a model compound. The 57Fe tracer experiments show that the Fe isotopes of the siderophores exchange readily with dissolved inorganic Fe. The results indicate that organic ligands are likely to be important in the generation of Fe isotope signatures in oxic environments. For example, the isotopic composition of marine Fe-Mn nodules may largely be due to the presence of strongly coordinating ligands.