In this study, 3-methylthiophene (3-mTh) was polymerized electrochemically in different ratios of boron trifluoride ethyletherate (BFEE) in acetonitrile (ACN) to investigate the effect of BFEE on impedance and polymer morphology. The monomer oxidation potentials were decreased by adding BFEE in ACN, from 1.25 V in 0% BFEE (0.1 M NaClO4-ACN) to 0.63 V in 15% BFEE-ACN, due to interaction (complexation) between the 3-mTh and BFEE during electrochemical polymerization of 3-mTh. Electrochemical Impedance Spectroscopic (EIS) experiments carried out in the electrolyte/poly(3-mTh)/ITO-PET electrodes in ACN solution in the monomer-free solution, indicated that as the ratio of BFEE was decreased, the capacitive behavior of the polymer was increased. The reduction products of BFEE and BFEE interact with positively charged oligomers by facilitating the deprotonation and resulting a lower oxidation potential. (C) 2014 Elsevier B.V. All rights reserved.