Geometric and electronic structures of some aniline derivatives have been obtained, aimed at a microscopic explanation of the observed electropolymerizability of these compounds. Monomers, dimers and their corresponding free radicals and ionic species, with the sulfonic acid group substituent located at various positions on the benzenoid ring, are considered. In this work we present an ab initio post-Hartree-Fock and density functional study of the ortho-, meta- and para-isomer of aminobenzenesulfonic acid (ortho-, meta-, and para-ABSA). To envisage a possible coupling scheme between ABSA molecules, we have obtained the equilibrium geometry and the electronic structure for monomers and dimers of ABSA as well as for the corresponding free radicals and ions. Based on the monomer-optimized geometries, atomic charges, bond orders and spin densities, a coupling scheme in the electrochemical polymerization is suggested. We have also calculated band gaps and ionization potentials. Some comments concerning the different theoretical methods used are made. (c) 2007 Elsevier Ltd. All rights reserved.