The non-compensated boron concentration in thin films of boron-doped polycrystalline diamond, chemically vapour deposited onto tungsten or silicon substrates, was determined by the methods of differential capacitance and amplitude demodulation. In the case of frequency-independent capacitance, both methods give practically the same concentration value. For samples with frequency-dependent capacitance, an upper estimate of the concentration was obtained (using both methods) of the same order of magnitude. In this case, the electrode impedance includes a constant-phase element.