The electropolymerization of methylthio-substituted oligothiophenes 2-4 has been studied. Theoretical and experimental results reveal that only 4,4＇-disubstituted derivatives yield polymeric films. The reason for this is that electron-donating methylthio (= methylsulphonyl) substituents in the ＇outer＇ beta-positions of the starting compounds 2-4, which cause high spin densities at the corresponding a-C atoms, favour fast coupling steps between the reacting oligomers during all stages of the electropolymerization. All data clearly indicate that the electropolymerization of donor-substituted thiophenes is not a chain propagation process but a series of successive ＇dimerization＇ steps. The resulting polymers have been characterized by voltammetric measurements, in situ conductivity experiments and spectroelectrochemistry.