Infrared spectroscopic ellipsometry for wave numbers from 333 to 4000 cm(-1) is demonstrated as a useful technique for characterization of structural, vibrational, and free-charge-carrier properties of conducting organic thin layers deposited on electrically conducting as well as on isolating substrates. Pentacene and poly(3,4-ethylenedioxy thiophene) /poly (styrenesulfonate) (PEDOT:PSS) are studied exemplarily. For both materials, the infrared dielectric functions are reported, which can serve as fingerprints for multiple-layer structures analysis. PEDOT:PSS layers deposited onto n-type and p-type silicon substrates reveal different conductivity response, whereas the IRSE data show that the pentacene layers on glass are rendered highly resistive. (C) 2003 Elsevier B.V. All rights reserved.