A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese( III) (Mn(III)TDMImP) is shown to react with H2O2 to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H2O2. The visible spectrum of a compound 0 type intermediate, assigned as Mn-III(OH)(OOH)TDMImP, can be directly observed under saturating H2O2 conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (Delta H-not equal=4.2 +/- 0.2 kcal mol(-1)) and a large, negative activation entropy (Delta S-not equal=-36 +/- 1 cal mol(-1) K-1). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 +/- 12 s(-1). These observations indicate that the O-O bond in Mn-III(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) -OH is partially deprotonated ("push"), while the terminal oxygen in -OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.