ZnO thin films of different thicknesses were deposited by pulsed direct-current magnetron sputtering onto poly(ethylene terephthalate) (PET) substrates and afterwards poly 3, 4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) was spin-coated onto the ZnO film. Spectroscopic ellipsometry in the Vis-fUV energy range (1.5-6.5 eV), X-ray diffraction and atomic force microscopy were used to reveal the properties of the deposited films. The size of crystallites increased from 5.1 to 7.4 nm, whereas the crystallinity of the ZnO films has been improved. The influence of different ZnO thickness on the optical properties of the PEDOT:PSS layer was studied as well. As the thickness of ZnO films increased. the surface roughness increased but the energy gap decreased after a critical thickness. Concerning the consequences to the PEDOT: PSS optical properties, no major changes occurred in the transition energies. (C) 2009 Elsevier B.V. All rights reserved.