A series of synthetic enzyme mimics of the dimanganese catalase enzyme from bacterial cells has been prepared and the chemical mechanism of their activity has been examined. These are formulated as [LMn2X]Y2, μ2-X = CH3CO2−, ClCH2CO2−, Cl−, OH−; Y = ClO4−, BPh4−, CH3COO−, Cl−, Br−, utilizing the septadentate ligand, HL = N,N,N′,N′-tetrakis(benzimidazole)-1,3-diaminopropan-2-ol. These catalyze the disproportionation of H2O2 into O2 and H2O by a mechanism indistinguishable from the enzyme mechanism. Like the enzyme three electrons can be removed from Mn to form four oxidation states ranging from Mn2(II,II) to Mn2(III,IV). The electrochemical properties, the energy level spacing between electronic spin states (S =0, 1..5), ligand exchange and disproportionation kinetics can be controlled by the choice of the bridging ligand X. The mechanism involves the cyclic oxidation and reduction of the Mn2(II,II) and Mn3(III,III) species by H2O. An important component of the kinetic barrier in the rate-limiting step appears to be the free energy required for oxidation to the Mn2(III,III) species. This is determined by the Mn ligands. An ENDOR study of the manganese catalase(III,IV) from T. thermophilus has revealed the Mn coordination environment to be predominantly carboxylate protein residues. This ligand environment greatly lowers the barrier to oxidation, enabling catalysis at rates approaching the diffusion limit.