A set of alpha-coupled oligothiophenes, dimer through hexamer, was studied. Each oligomer was terminated with two methyl groups and was substituted with two or four methoxy groups. Electrochemical studies showed that methoxy groups substituted at "inside" beta-positions on the terminal rings (positions toward the center of the chain) gave stable cation radicals. The E(0) values for the reversible couples were 0.24-0.52 V (SCE). The changes in E(0) with structure were explained using PPP molecular orbital calculations. Cation radicals were generated using either chemical or electrochemical oxidation, and in several cases stable salts were isolated. The vis-near-IR spectra of the cation radicals were recorded. In CH2Cl2 solvent the cation radicals were monomeric and showed two pi-pi*-bands, which shifted to longer wavelength with oligomer length. In CH3CN solvent the cation radicals were primarily in the form of pi-dimers, which showed two pi-pi*-bands and a charge transfer band at longer wavelength in the near-IR. All three bands shifted to longer wavelength with oligomer length. The equilibrium constant for dimerization increased by a factor of 10(3) on going from a terthiophene to a quinquethiophene. Spectra of oligomers terminated with hydrogens or dimethylphenylsilyl groups showed that steric effects of the small hydrogen and bulky silyl group on pi-dimerization were very small. Methyl-terminated oligomers substituted with "outside" methoxy in the terminal rings gave rather positive oxidation potentials and unstable cation radicals. In one case, the cation radical decomposition products were studied and it was shown that reactions had occurred on a terminal methyl group.