Mononuclear non-haem iron(III)-superoxo species (Fe III -O 2 '·) have been implicated as key intermediates in the catalytic cycles of dioxygen activation by non-haem iron enzymes. Although non-haem iron(III)-superoxo species have been trapped and characterized spectroscopically in enzymatic and biomimetic reactions, no structural information has yet been obtained. Here we report the isolation, spectroscopic characterization and crystal structure of a mononuclear side-on (• 2) iron(III)-superoxo complex with a tetraamido macrocyclic ligand. The non-haem iron(III)-superoxo species undergoes both electrophilic and nucleophilic oxidation reactions, as well as O 2 -transfer between metal complexes. In the O 2 -transfer reaction, the iron(III)-superoxo complex transfers the bound O 2 unit to a manganese(III) analogue, resulting in the formation of a manganese(IV)-peroxo complex, which is characterized structurally and spectroscopically as a mononuclear side-on (• 2) manganese(IV)-peroxo complex. The difference in the redox distribution between the metal ions and O 2 in iron(III)-superoxo and manganese(IV)-peroxo complexes is rationalized using density functional theory calculations.