Geometries of the ground state and energies of the lower-lying excited singlet states of phosphorus-containing porphyrins were computed using DFT and TD-DFT methods, respectively, and compared with those of N-porphyrins. The P-analogues of porphyrins were obtained by successively replacing a NH- by a PH-unit or a N- by a P-atom within N-porphyrins. The goal of this theoretical study is to elucidate the modifications in the electronic structure and to predict the shift of Q- and B-bands in the UV-VIS absorption spectrum of porphyrins following substitution of nitrogen by phosphorus atoms. The interesting features could be of help in the identification of the P-substituted porphyrins that are not known yet. (C) 2003 Elsevier B.V. All rights reserved.