DFT calculations have been performed on a series of P-substituted copper porphyrins, CuP-X, with particular regard to substituent effects upon the relative energies of two frontier occupied, nearly degenerate a(2u) and a(1u) orbitals. Substitution by electron-withdrawing groups only slightly enlarges the a(2u)-a(1u) separation of CuP. Electron-donating groups CH2OH and OCH3 raise the energies of a(2u) and a(1u) uniformly, In contrast, SH and NH2 reverse the normal ordering; these [CuP-X](+) cation radicals are mainly a(1u) in character. Electronic structures of CuP derivatives substituted at all eight P-pyrrole positions are also examined. (C) 2002 Published by Elsevier Science B.V.