Quantum-chemical calculations are performed to study the geometric and electronic structures of poly(3,4-ethylenedioxythiophene) - PEDOT. The relative stability of the two possible structures for PEDOT (aromatic-like and quinoid-like) has been evaluated on oligomers of increasing size. The results obtained on PEDOT are compared to those collected on polythiophene and polyisothianaphthene, i.e., two parent conjugated polymers that are known to possess an aromatic and quinoid ground state, respectively. The vibrational spectra of both forms of PEDOT have also been calculated and compared with recent experimental data. The calculations indicate that the ground state of neutral PEDOT is aromatic-like. (C) 2002 Elsevier Science B.V. All rights reserved.