The electrochemistry of water-soluble iron porphyrins (Fe(n-TMPyP)) (where n = 2 and 4) was studied as an electrochemically active film on DNA modified glassy carbon, gold, platinum, and transparent semiconductor tin oxide electrodes in solutions of various pH values. The two layers of the modified electrode containing the iron porphyrin and the DNA film were prepared by depositing the iron porphyrin on a DNA film modified electrode. The Fe(4-TMPyP)/DNA film was electrocatalytic reductive for p nitrobenzoic acid in a weak acidic, or neutral aqueous solution through an Fell species, and the electrocatalytic reduction peak potential became more negative than the cathodic peak of the Fem(III/II) redox couple. The electrocatalytic reduction properties by the Fe(2-TMPyP)/DNA film as catalysts for nitrite reduction have also been determined, and shown to be active through an Fe-I species and to be pH-dependent. The electrocatalytic oxidation properties of nitrite by Fe(n-TMPyP)/DNA (for it = 2 and 4) film have also been determined and shown to be active through an Fe-IV species with the electrocatalytic oxidation efficiency of NO2- with Fe-IV(O)(n-TMPyP) being higher than with (HO)Fe-IV(O)(n-TMPyP). The electrocatalytic oxidation efficiency of NO2- by iron porphyrin is pH-dependent. The electrocatalytic reduction of p-nitrophenol by Fe(2-TMPyP)/DNA film are also discussed. (C) 2003 Elsevier Ltd. All rights reserved.