Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(4-styrenesulphonate) (PSS) has been widely used in light-emitting devices as hole transport layer and in photovoltaic devices as hole-collecting layer. In the present study, various quantum chemical calculations were carried out for the investigation of low-lying excited states of ethylenedioxythiophene (EDOT), and styrenesulphonic acid. The lowest adiabatic transition energies were calculated using configuration interaction singles method. The time-dependent density functional theory was also applied for the calculation of the vertical excitation energies. Differential self-consistent-field-based density functional theory method is well known to show good performance for the geometry of excited state and hence it was also applied to study of the first singlet excited state. In addition to the calculation of the monomer, the electronic properties of PEDOT were calculated by periodic density functional theory method and the result is in good agreement with the experimental observation. (c) 2004 Elsevier B.V. All rights reserved.