The electrochemical behaviour of Ta-TaOx structures was investigated by impedance spectroscopy. It was shown that, after the polishing treatment, the electrode is covered by an "initial" oxide film containing tantalum monoxide, TaO, and that "electrogenerated" oxides are only composed of tantalum pentoxide, Ta2O5 (greater than or equal to 97 mol%). However, the dielectric properties of the structures strongly depend on the thickness of the oxide, d(ox), because of the influence of a sub-stoichiometric oxide of TaO in thin layers, in agreement with previous results obtained by XPS measurements. The variation of the reciprocal total capacitance of the oxide, C-ox, with the quantity of electricity, Q(a), involved in the formation of the oxide exhibits two linear parts. The breakdown of the slope of that curve is interpreted by a variation of the relative permittivity, epsilon(r), of the oxide with its thickness : for thin films (d(ox) less than or equal to 19 nm), epsilon(r) approximate to 18.5 and for thick films (d(ox) > 19 nm) epsilon(r) approximate to 27.5.