The absorption pattern of infrared active vibrations (IRAVs) of p- and n-doped thiophene based conjugated polymers with low electronic band gaps are compared. The polymers were doped electrochemically and studied in situ with attenuated total reflection Fourier transform infrared spectroelectrochemistry. Substantial spectral differences were found for polyethylenedioxythiophene (PEDOT), a polyisothianaphthenemethine derivative (PIM) and two polydithienothiophenes (PDTT1 and PDTT2), whereas PDTT3 has no significant differences in the IRAV spectra. The results are related both to structural effects with selective participation of different chain units (for PIM) and to different localisation of the doping induced charge carriers of both signs (for PEDOT, PDTTl and PDTT2). (C) 2003 Elsevier B.V. All rights reserved.