Spectroscopic and electronic structure studies of oxidized manganese catalase (OxMnCAT) and mu-oxo-di-mu-carboxylato-bridged dimanganese(III) model complexes are presented and discussed, providing, for the first time, a detailed experimental description of the catalytically relevant Mn(III)/Mn(III) binuclear active site of MnCAT. The absorption and MCD spectra of the synthetic complexes are similar to one another, indicating that the transitions below 30 000 cm(-1) are inherent to the [(Mn2O)-O-III(OAc)(2)](2+) core. A combination of Raman excitation profile and polarized single-crystal absorption measurements on [Mn2O(OAc)(2)(Me(3)tacn)(2)]-(ClO4)(2) . H2O and density functional electronic structure calculations are used to identify the key spectral features of the model complexes and their relation to the dominant Mn-O(oxo) bonding interaction in these dimers. On the basis of these results, analysis of the spectroscopic data of OxMnCAT leads to the proposal that the active site consists of two ferromagnetically coupled five-coordinate Mn(In) ions linked by a hydroxo bridge, contrasting previous proposals of an oxo bridge at this site. The interaction of OxMnCAT with azide, an electronic structural mimic of H2O2, has been studied using absorption and MCD spectroscopies. It appears that azide binds to both Mn(III) ions of the active site by displacement of one ligand on each metal, indicating that both ions are available for reaction with H2O2 The significance of our results with respect to the catalytic peroxide disproportionation reaction accomplished by MnCAT is discussed.