We report unique in situ conductance results with comparable conductance values in p- and n-doping ranges for a series of poly(3,4-dialkoxythiophenes). The polymers with different lengths of alkyl chain in the side alkoxy groups (1, 3 and 8 carbon atoms) were synthesized electrochemically from acetonitrile solutions of monomers on arrays of Pt microelectrodes or on a two-band Au electrode and studied in acetonitrile solution of 0.1 M TBAPF(6). The differences in the height of the conductance responses in the n-doping range as well as in the width of a high conductance window for oxidized polymers were discussed in terms of different conjugation lengths resulting from different lengths of substituents. The results obtained in the p- and n-doping ranges and the STM images of poly(3,4-dioctyloxythiophene) film were compared with those for poly(3,4-ethylenedioxythiophene) (PEDOT). The electrochemical stability of poly(3,4-dialkoxythiophenes) at different oxidation potentials was studied by in situ UV-Vis spectra recorded during double potential step experiments. (c) 2004 Elsevier B.V. All rights reserved.