Quantum mechanical calculations at the density functional theory level have been used to examine the interaction between 3,4-ethylenedioxy-thiophene and N-methylpyrrole units when they are directly linked in copolymer chains. Specifically, in this study, which was motivated by experimental observations of electrochemically synthesized copolymers, the conformation and electronic properties of co-oligomers formed by two, three and four units have been examined in both neutral and radical cation states. Results reveal significant differences, especially in the radical cation state, between the co-oligomers and the corresponding individual homo-oligomers, which have also been explored considering the same theoretical method. Furthermore, electronic properties of oligomers have been used to estimate those of poly (3,4-ethylenedioxythiophene), poly(N-methylpyrrole) and the copolymer with alternated structure, which have been compared among them. The overall results allowed to understand the anomalous features detected in copolymers prepared by electrochemical techniques from 3,4-ethylenedioxythiophene and N-methylpyrrole mixtures, which were initially attributed to some type of unfavourable interaction between the two types of monomeric units. (C) 2007 Elsevier Ltd. All rights reserved.