The electrochemical behavior of polycrystalline diamond films of different thickness (0.5-7 mu m), grown by hot-filament CVD method, was studied by electrochemical impedance spectroscopy and cyclic voltammetry. The differential capacitance, background current, and potential window were measured in supporting electrolyte solution; the electrochemical kinetics, in [Fe(CN)(6)](3-/4-) model redox system. With the increasing of the films thickness, the crystallite size increased; both the differential capacitance and background current in the indifferent electrolyte, as well as the transfer coefficients in the redox system, decreased; thus, the diamond electrode becomes as if less reversible. The effect of the films' thickness is reduced to that of nondiamond (amorphous) carbon contribution from intercrystalline boundaries on the electrochemical behavior of the polycrystalline diamond electrodes.