Different metalloporphyrin model compounds have been synthesized to study the mechanisms of cytochrome P450s with various terminal oxidants, and numerous intermediates have been reported. However, the detailed mechanism of the oxygen atom transfer from iodosylarene to the substrates remains unclear. Here we report the direct ultraviolet-visible spectroscopic observation of the soluble iodosylarene-manganese porphyrin adduct following catalytic oxidation using 2,4,6-tri-tert-butylphenol as the reductant. When the reductant is changed to cis-stilbene, the rate-determining step also changes. Both the iodosylarene-manganese porphyrin adduct and [(porphyrin)Mn(V)=O] species may be simultaneously observed. In the absence of reductant, the adduct of iodosylarene with sterically hindered [Mn(meso-tetrakis(2,6-dichlorophenyl)porphinato)Cl] is immediately formed, and smoothly converted into a high-valent [(porpyrinato)Mn=O]. Electrospray ionization mass spectrometry analysis of the reaction further confirms the transformation between these species. This study provides an insight into the mechanism of oxygen transfer within the haem-containing enzymatic systems.