Broken-symmetry density functional theory was used to study the catalytic center of manganese catalase in the superoxidized Mn(III)/Mn(IV) state. Heisenberg exchange coupling constants, 55Mn and 14N hyperfine coupling constants (hfcs) and nuclear quadrupole splittings, as well as the electronic g tensors were evaluated for different model systems of the active site after complete geometry optimizations in the high-spin and broken-symmetry states. A comparison of the experimental data with the spectroscopic parameters computed for the models with unprotonated and protonated mu-oxo bridges shows best agreement between theory and experiment for a Mn2(mu-O)2(mu-OAc) core. The calculated Mn-Mn distances and 55Mn hfcs clearly support a dimanganese cluster with unprotonated mu-oxo bridges in the superoxidized state. Furthermore, it is shown that an interchange of the Mn(III) and Mn(IV) oxidation states in this trapped valence system leads to specific changes in the molecular and electronic structure of the manganese clusters.