The polymerization of 3-hexylthiophene with FeCl3 was carried out in aromatic solvents. The incorporation of aromatic solvent molecules such as benzene, toluene, xylene, and mesitylene into the resultant polymer main chain was clarified by H-1 NMR and matrix-assisted laser desorption/ionization mass spectrometry for the first time. More amount of benzene (12 mol%) was incorporated into the resultant polymer, compared with toluene, xylene, and mesitylene. The polymeric product prepared in benzene at 23 degrees C afforded an electrical conductivity (0.3 S cm(-1)) much better than that of the polymer prepared in toluene (0.05 S cm(-1)).