Sexi-, novi-, duodeci- and quindecithiophenes with hexyl groups were prepared by a reductive coupling reaction of dibromodihexylterthiophene with an activated nickel(0) reagent and purified by liquid chromatography. The structures were determined by H-1 n.m.r. spectra, g.p.c. data, and elemental analyses. The comparison of their pi-pi* transitions with those of poly(alkylthiophene)s showed that the polymers had a similar effective conjugation length to the novithiophene derivative. The oligothiophenes were oxidized with stoichiometric amounts of FeCl3 in methylene chloride and characterized by u.v.-vis.-n.i.r. absorption and e.s.r. spectroscopy. The first oxidation of the oligothiophenes yielded cation radicals (polarons), which were spread over thiophene rings between 6 and 8. The absorption spectra of the first-oxidized oligothiophenes were similar to those of the poly(alkylthiophene)s oxidized in solution. The further oxidation of the oligothiophenes produced a heavily oxidized species, such as dications (bipolarons), trication radicals, or poly(cation radical)s. The absorption spectra of the heavily oxidized oligothiophenes were quite different from those of the oxidized poly(alkylthiophene)s. These results indicated that oxidized species in poly(alkylthiophene) solutions were also cation radicals. Furthermore, the absorption spectra of the oxidized quindecithiophene derivative suggested the presence of the cation radical in the second-electron oxidized state.