The potentiodynamic oxidation of Zr at upsilon = 0.05 V s-1 was investigated in the potential range 0-90 V in the following electrolytes : H2SO4, HNO3, H3PO4 and NaOH. After film growth, the oxides were characterized by ac impedance measurements in the frequency range 0.1 Hz less-than-or-equal-to f less-than-or-equal-to 10 kHz. The impedance responses of the films formed in H2SO4 and NaOH were modelled with a transfer function corresponding to a single-layer model. The impedance spectra of the oxides formed in H3PO4 and HNO3 showed two time constants which were adequately simulated by a transfer function corresponding to a two-layer oxide structure. The nature of the outer layer of the films formed in H3PO4 was attributed to phosphate incorporation form the electrolyte, which was confirmed by X-ray photoelectron spectroscopy measurements. The variation of the oxide electrical parameters (resistance, capacitance etc.) with final formation potential is reported and the reasons leading to the complex film structure are discussed.