Variable-field, variable-temperature saturation magnetization experiments have been used to derive structural information about the catalytically active Mn(III)Mn(III) state of the Thermus thermophilus catalase. They reveal that oxidized catalase contained not one but two Mn(III)Mn(III) forms which are in a pi-I-dependent equilibrium as illustrated in the Scheme. The low-pH form is proposed to contain an oxo and carboxylato bridge Mn(III)Mn(III) center. The high-pH form is proposed to contain a di(mu-oxo)(mu-carboxylate)dimanganese(III).