Addition of H+ to a synthetic (mu-1,2-peroxo)diiron(III) model complex results in protonation of a carboxylate rather than the peroxo ligand. This conclusion is based on spectroscopic evidence from UV-vis, Fe-57 M-0 ssbauer. resonance Raman. infrared, and H-1/F-19 NMR Studies. These results Suggest a similar role for protons in the dioxygen activation reactions in soluble methane monooxygenase and related carboxylate-bridged diiron enzymes.