The dichloride-bridged [TPA 2Mn 2(μ-Cl) 2] 2+ complex ( I) was synthesized as a structural and functional model complex of the chloride-inhibited manganese catalase, and its catalytic properties in MeCN have been studied. Complex I shows sigmoidal kinetics and the activity is significantly inhibited in the presence of water. The kinetic parameters of the hydrogen peroxide disproportionation by complex I have been successfully fitted with non-Michaelis–Menten kinetics of Hill’s equation, which implies a multiple-step substrate activation of complex I. After termination of the catalysis, the mononuclear [TPA 2Mn](ClO 4) 2 was isolated from the solution. During the catalysis, a new penta-coordinate [TPAMnCl](ClO 4) complex ( IV), tending to accumulate at a lower ratio of H 2O 2, was isolated and its X-ray crystallographic structure, as well as physical properties, was determined. Transformation of complex I in the presence of different molar ratios of H 2O 2 was studied by UV–Vis, EPR and ESI-MS spectroscopy. Upon addition of H 2O 2, the catalytic solution turned dark green, with instant evolution of oxygen gas, and the electronic spectra obtained were identical to that of the dark green dioxo bridged [TPA 2Mn 2(μ-O) 2](ClO 4) 3 complex ( III). When the catalytic solution was subjected to EPR measurement, the transient peaks corresponding to the electronically localized Mn(II) species developed in a short time at lower concentrations of H 2O 2. The signal was more distinctive in the presence of water, and the complex I·H 2O 2 adduct was suggested as the intermediate species based on ESI-MS measurements. The EPR signal corresponding to complex III was detected at higher concentrations (>800 equiv.) of H 2O 2. As possible catalytic intermediates, the [TPA 2Mn III 2(μ-O) 2] 2+ and [TPA 2Mn III 2(μ-O)(μ-OH)] + species were suggested. A possible catalytic mechanism of H 2O 2 disproportionation by complex I, including the formation of active species and termination of the catalysis, has also been suggested.