Electrochemical impedance spectroscopy was used to investigate the electrochemical polymerization of bithiophene under galvanostatic conditions. The impedance spectra obtained on layers with different thicknesses were analyzed by a fitting procedure, based on theoretical models known from the literature. It was shown that the model of Ho et al. (C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Soc., 127 (1980) 343) can be applied successfully in studying the impedance spectra of relatively thin polybithiophene layers, deposited at low current density. The model of Paasch et al. (G. Paasch, K. Micka and P. Gersdorf, Electrochim. Acta, 38 (1993) 2653) in a simplified form is better suited to describe the spectra of thicker, supposedly porous polymer layers, obtained at higher current density. It was suggested that the deposition of polymer film onto a metal electrode follows a 3D growth mechanism during the first stages of polymerization, and pseudo-2D growth for polymerization at charges higher than 15 to 20 mC cm(-2). Important parameters of the polymer film, such as the apparent diffusion coefficient, apparent diffusion length, ionic conductivity and polymer layer thickness were estimated.