The electrochemical oxidation of Ta and Nb and the dielectric behaviour of the oxide films thus formed were investigated in the following electrolytes: H2SO4, HNO3, H3PO4 and NaOH. Characterization of the films was carried out by means of potentiodynamic current-potential profiles tin the range 0-8 V) and electrochemical impedance spectra tin the range 0.1 Hz-100 kHz). The a.c. response of the oxide films was modelled as a single layer structure on the basis of an equivalent circuit with constant phase elements (CPE). The dependence of the oxide resistance and oxide capacitance with potential is also reported.