The spectroelectrochemical behaviour of cast poly(3-octylthiophene) (POT) films (0.2 and 0.5 mu m) on indium-tin-oxide (ITO) glass electrodes has been investigated in organic media. These thin films exhibit interesting electrochromic properties and their application in electrochromic devices has been examined in liquid (CH3CN + LiClO4 0.3M) and viscous electrolyte (PEO + CH3CH + LiClO4). Polyaniline (PANI) him appears to be a convenient complementary counter electrode since its transmission maximum corresponds to its oxidized state and that of POT film to its reduced state. Thin films of iridium oxide (IrO2) are also possible counter electrodes, even in acetonitrile, the electrochemical behaviour being mostly capacitive with a low transmission change. However, the best contrast is obtained with the POT/PANI system.