The influence of the polarity of the medium on the electronic properties and saturation behavior of pi-conjugated oligomers and polymers based on thiophene have been investigated. Quantum mechanical calculations in the gas-phase and both acetonitrile and water solutions have been performed considering the neutral and positively charged forms of oligomers made of thiophene, 3,4-ethylenedioxythiophene and 3,4-phenylenedioxythiophene. Results indicate that the influence of the medium on the HOMO-LUMO gap is practically negligible while it affects considerably both the vertical and adiabatic ionization potentials, even though the magnitude of such effects depends on the chemical structure of the repeating unit. (C) 2011 Elsevier B. V. All rights reserved.