Methylated bi- and terthiophenes (monomers, 1, 3, 5, 8) with one free alpha-position can be oxidized with FeCl3 . 6H(2)0. An irreversible dimerization reaction forms dimers of the expected structure. The radical cations of the monomers also react with free beta-positions of the formed dimers (6, 9) having two methylated alpha-positions. The products of these reactions show alpha,beta’ linkages (7, 10, 11). Different beta-positions in end-capped dimers show a graduated reactivity against radical cations of the monomers; the free beta-position with the higher electron spin density can be attacked preferably by an alpha-position of a radical cation of a monomer molecule. No products with beta,beta’ linkages were found. Electrochemical investigations using fast-scan voltammetry show the formation of alpha,alpha,’ linkages of methylated oligothiophenes. For bithiophene derivatives, rate constants of the dimerization process in the range of 10(9) L . mol(-1). s(-1) were determined. In comparison with a terthiophene derivative, it was found that the dimerization rates decrease with increasing chain length of the oligomers.