We have generated a high-spin Fe-III-OOH complex supported by tetramethylcyclam via protonation of its conjugate base and characterized it in detail using various spectroscopic methods. This Fe-III-OOH species can be converted quantitatively to an Fe-IV=O complex via O-O bond cleavage; this is the first example of such a conversion. This conversion is promoted by two factors: the strong Fe-III-OOH bond, which inhibits Fe-O bond lysis, and the addition of protons, which facilitates O-O bond cleavage. This example provides a synthetic precedent for how O-O bond cleavage of high-spin Fe-III-peroxo intermediates of non-heme iron enzymes may be promoted.