Poly3-octylthiophene (P3OT) was synthesized by direct oxidation of 3-octylthiophene using FeCl3 as oxidant/catalyst. It wag observed that the purity of FeCl3 affected the molecular weight and polydisparity as well as the degree of regioregularity of the polymer, measured by size exclusion chromatography and by H-1 nuclear magnetic resonance, respectively. Optical absorption proper-ties of the P3OT films were analyzed as a function of solution concentration, the type of the substrates and the mole percentage of the doping agent. Morphology study with atomic force microscopy indicates that the surface roughness and the cluster size of the polymeric films are influenced by the P3OT solution concentration and the percentage molarity of ferric chloride. FeCl3 doped P3OT films show enlarged clusters compared with the pristine one due to the introduction of FeCl3 Molecules in the original P3OT chains, a phenomenon observed in electrochemically doped heterocyclic conducting polymers reported in the literature. (c) 2005 Elsevier B.V. All rights reserved.