An in situ spectroelectrochemical study was carried out on the formation of 3-methylthiophene oligomer radical-cations (monomer, dimer, and trimer) and the 3-methylthiophene dihydrocation in the solution bulk during the electrochemical synthesis of a poly-3-methylthiophene coating on a conducting glass electrode. A dependence of the concentration of products formed on the cation radius (Li + and Na + ) and nature of the anion (ClO 4 -and BF 4 -) of the base electrolyte was established. The conversion of charged components into neutral dimers and tetramers of 3-methylthiophene after termination of the anodic polymerization was shown in accord with reported results on the electrochemical polymerization of thiophenes.The rather good results obtained in the use of poly(3-alkylthiophene), including poly-(3-methylthiophene) (P3MT) in photovoltaic, sensor, and electrochemical devices [1] is a function not only of the current level of understanding of the physical and chemical aspects of the nature of the polymer but also of the high quality of the polymers obtained. The mechanisms of the chemical and electrochemical polymerization of their polymers have been described in rather considerable detail in the literature [2][3][4]. In particular, the electrochemical polymerization of 3-alkylthiophenes is known to proceed through a radical-cation mechanism with formation of long polyconjugated chains [3]. The polymer is obtained in an oxidized state, in which the positive charge is compensated by the base electrolyte anions. Thus, the structure and properties of the resultant poly(3-alkylthiophenes) should depend on the nature of these anions [5]. We should note that while the effect of these anions on the electrochemical polymerization and properties of the resultant polymer is rather clear and has been extensively studied, the functions of the electrolyte cations in such systems have not received adequate attention as a consequence of their apparent insignificance. However, in the case of the galvanostatic electrochemical polymerization of 3-phenylthiophene (3PT), Pokhodenko and Krylov [6] found that overvoltage of this process drops in the cation series Bu 4 N + > Et 4 N + > Na + > Li + . The oxidation potential of 3PT decreases in virtually the same series (without Et 4 N + ) [7]. According to Pokhodenko et al. [6,7], such an effect of the base electrolyte cations on the electrochemical stage of the process is related to the interaction of these cations with the sulfur atom of the uncharged aromatic thiophene ring. Furthermore, the electrochemical polymerization of 3MT is accompanied by a transition of the intermediate 3MT oligomer radical-cations to the solution bulk from the anode surface (where oxidation of the monomer occurs). These charged intermediates subsequently undergo chemical reaction with 158 0040-5760/10/4603-0158