A stepwise catalytic oxidation for nitric oxide and nitrite by water-soluble iron(III) meso-tetrakis(N -methylpyridinium-4yl)porphyrin (Fe(4-TMPyP)) was first revealed by using an ITO electrode. Electrochemical and spectroelectrochemical studies characterized the formation of oxoiron(IV) porphyrin and oxoiron(IV) porphyrin p-cation radical in the oxidation of Fe(4TMPyP). O@Fe(4-TMPyP) showed se...