Catalytic oxidation of nitric oxide and nitrite by water-soluble manganese(III) meso-tetrakis(N-methylpyridinium-4-yl) porphyrin (Mn(III)(4-TMPyP) was first studied at an indium-tin oxide (ITO) electrode in pH 7.4 phosphate buffer solutions. A stepwise oxidation of Mn(III)(4-TMPyP) through high-valent manganese porphyrin species has been observed by electrochemical and spectroelectrochemical (OTTLE) techniques. The formal potential of 0.63 V for the formation of O=Mn(IV)(4-TMPyP) has been estimated from OTTLE data. The product, oxoMn(IV) porphyrin, was relatively stable decaying slowly to Mn(III)(4-TMPyP) with a first-order rate constant of 3.7 x 10(-3) s(-1). O=Mn(IV)(4-TMPyP) has been found to oxidize NO catalytically at potentials about 70 mV more negative than that previously reported for O=Fe(lV)(4-TMPyP) with good selectivity against nitrite. Nitrite was catalytically oxidized at potentials higher than 1.1 V presumably by O=Mn(V)(4-TMPyP). O=Mn(IV)(4-TMPyP) was observed as an intermediate species. Nitrate has been confirmed to be a final product of the electrolysis at 1.2 V, while at 0.8 V nitrite left unchanged, demonstrating that O=Mn(IV)(4-TMPyP) could not oxidize nitrite. A possible schemes of the catalytic oxidation of NO by O=Mn-IV(4-TMPyP) and NO2- by O=Mn(V)(4-TMPyP) have been proposed. (C) 2005 Elsevier Ltd. All rights reserved.