Abstract: As models for compound I species of heme monooxygenases such as cytochrome P450, chloroperoxidase and myeloperoxidase, the oxoMn(V) porphyrin transients for four cationic porphyrins TM-2-PyP, TM-4-PyP, TF₄TMAP and TDMImP were studied by fast mixing stopped-flow spectrophotometry in aqueous solution. In particular, oxoMn(V)TM-2-PyP is stable for several minute, allowing its characterization by 1H NMR. This intermediate was assigned as a low spin d2 complex based on the similarity of its spectra to that of diamagnetic TM-2-PyP complexes. The reactivities of the oxoMn(V) porphyrin complexes were characterized by three types of reactions: (i) electron transfer from nitrite, (ii) oxygen atom transfer to halides and olefins, (iii) hydrogen abstraction from phenolic O-H bond and benzylic C-H bond. The more electron rich oxoMn(V) porphyrins react faster for all three types of reactions, opposite to typical structure-reactivity relationship. The fast and reversible oxygenation of bromide, in particular, has provided an opportunity to investigate this unusual correlation by a combined experimental and theoretical approach. The kinetic inertness of the more electron deficient oxoTM-2-PyP was ascribed to both the lower basicity of its oxo ligand, and the higher low-spin to high-spin promotion barrier, compared to that of the isomeric oxoMn(V)TM-4-PyP. The reaction between these water soluble MnIII porphyrins and H₂O₂ was shown to have Michaelis Menton type saturation kinetics. An intermediate, assigned as MnIII(OH)(OOH) can be directly observed under saturated H₂O₂ conditions for TDMImP. The rate determining step is found to be pH-independent with a large negative activation entropy. Based on these observations, we proposed that the distal O-O bond is cleaved by a strong 'push' from the oxygen anion formed by releasing the proton in axial OH- ligand. This 'push' effect, similar to the thiolate push found in cysteinate heme monooxygenases, can be attributed to the electron repulsion between manganese d electrons and negative charges on the axial oxygen.