This is an electrochemical study of the Ni(II) and Cu(II) complexes of a series of beta -octahalogeno-meso-tetrakis(p-X-phenyl)porphyrins, M[Y8T(p-X-P)P] (M = Ni, Cu; Y = Cl, Br; X = CH3, H, F, Br, COOMe, CF3, NO2). We have analyzed the oxidation and reduction potentials by means of the Hammett equation. The Ni and Cu series exhibit significant differences in electrochemical behavior in terms of the absolute values of the redox potentials and substituent effects. DFT calculations suggest that these differences may be ascribed to differences in saddling-induced metal(d(x2-y2))-porphyrin(a(2u)) orbital interaction between the Ni and Cu porphyrins.