3,4-Ethylenedioxythiophene and 3-octylthiophene were electropolymerized on glassy carbon electrodes (GCE) to compare with four different electrolytes [lithium perchlorate (LiClO4), sodium perchlorate, tetraethylammonium tetrafluoroborate, and tetrabutylammonium tetrafluoroborate] in a solvent of acetonitrile (CH3CN). Modified electrodes were characterized by cyclic voltammetry, attenuated total reflectance-Fourier transform IR spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, atomic force microscopy, and electrochemical impedance spectroscopy (EIS). Nyquist and Bode plots for magnitude, phase, admittance, and capacitance on both polymer-modified electrodes were comparatively investigated in detail. The highest low-frequency capacitance (C (LF)) and double-layer capacitance (C (dl)) were obtained in 0.1 M LiClO4/CH3CN for poly(3,4-ethylenedioxythiophene) and poly(octylthiophene)/GCE. EIS data were fitted to the equivalent circuit model of R(Q(R(C(R(C(RW))))))(CR), which is used to investigate circuit parameters.