A stepwise catalytic oxidation for nitric oxide and nitrite by water-soluble iron(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (Fe-III(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 pi-cation radical in the oxidation of Fe-III(4-TMPyP). O=Fe-IV(4-TMPyP) showed selective catalytic oxidation towards NO against NO2- with a normalized catalytic current of 3.4. The electrogenerated O=Fe-IV(4-TMPyP) pi-cation radical effectively catalyzed the oxidation of NO2- to NO3-via nitrogen dioxide ((NO2)-N-.) intermediate. Product analysis clarified that nitrite was the sole product in the catalytic oxidation of NO by O=Fe-IV(4-TMPyP) electrogenerated at 0.85 V vs. Ag\AgCl, but nitrate was the final product after electrolysis at 1.1 V vs. Ag\AgCl. The different chemical reactivities of high-valent iron porphyrins electrogenerated at the separated potentials indicated the possibility of selective electrochemical detection of NO. A mechanism of typical chemical catalysis was proposed for the catalytic oxidations of NO and NO2- by oxoiron(IV) porphyrin complexes. (C) 2004 Elsevier B.V. All rights reserved.