The disproportionation reaction of oxoiron(IV) porphyrin complex (II) to oxoiron(IV) porphyrin pi-cation radical complex (I) and iron(III) porphyrin complex (III) have been proposed in various reactions. However, there have been no report that clarifies the disproportionation reaction spectroscopically. Here, we show direct evidence for the disproportionation reaction of II with absorption, H-2 NMR, and EPR spectroscopy. Kinetic study of the disproportionation reaction with stopped flow technique can be analyzed as the second-order reaction for the concentration of proton and the first-order for the concentration of II. These results allow us to propose the mechanism of the disproportionation reaction, involving the sequential addition of two protons to the oxo ligand of II, to give an iron(III) porphyrin pi-cation radical species, which reacts with another II to afford I and III.