The cytoplasmic labile iron pool supplies iron to the mitochondrion for heme and iron sulfur cluster synthesis and to many cytoplasmic enzymes, thereby controlling numerous metabolic reactions. Surprisingly the chemical nature of this pool has never been convincingly characterised. Here we provide evidence for iron(II)glutathione being the dominant component of this pool. We report for the first time the affinity constant for the glutathione-iron(II) interaction and use this value to study the cytoplasmic speciation of iron(II). The formation of this complex is a major determinant of the electrode potential of the cytoplasmic ferrous iron pool, a means of selecting between iron(II) and manganese(II) and it provides a substrate for glutaredoxin/iron clusters at the dimer interface of glutaredoxins involved in the synthesis of Fe-S cluster proteins.