Oxygenation of a diiron(II) complex, [Fe-2(II)(mu-OH)(2)(BnBQA)(2)(NCMe)(2)](2+) [2, where BnBQA is N-benzyl-N,N-bis(2-quinolinylmethyl)amine], results in the formation of a metastable peroxocliferric intermediate, 3. The treatment of 3 with strong acid affords its conjugate acid, 4, in which the (mu-oxo)(mu-1,2-peroxo)diiron(III) core of 3 is protonated at the oxo bridge. The core structures of 3 and 4 are characterized in detail by UV-vis, Mossbauer, resonance Raman, and X-ray absorption spectroscopies. Complex 4 is shorter-lived than 3 and decays to generate in similar to 20% yield of a diiron(III/IV) species 5, which can be identified by electron paramagnetic resonance and Mossbauer spectroscopies. This reaction sequence demonstrates for the first time that protonation of the oxo bridge of a (mu-oxo)(mu-1,2-peroxo)diiron(III) complex leads to cleavage of the peroxo O-O bond and formation of a high-valent diiron complex, thereby mimicking the steps involved in the formation of intermediate X in the activation cycle of ribonucleotide reductase.