The aim of the present work is to analyse the specific contribution of the differential capacitance of SiO₂ ultra-thin layers to the impedance diagrams of Si/Oxide/Electrolyte (SOE) structures. In usual techniques dealing with MOS devices, the determination of the capacitance/voltage characteristics in MOS devices is hindered by the high value of the tunneling leakage current. In this work, the difficulty was overcome by careful measurement of the impedance diagrams using a SOE structure, under zero current flow. With this novel technique we obtained one RC equivalent circuit when the bias potential corresponded to the accumulation regime, whereas two well separated circuits appeared under the depletion regime. An interesting feature of the method is that both R and C components were derived from the data processing. It is known that the measured value of the oxide layer capacitance is sensitive to the charging process of the space charge layer. We observed that the experimental values of the depletion layer capacitive term were in the range of a few 10⁻² μF cm⁻². These results were consistent with a theoretical treatment of the charge distribution near the flatband potential. In the case of a thermal oxide insulating layer a few nanometers thick, the capacitance was found equal to a few μF cm⁻², in agreement with the computed value from the SiO₂ oxide thickness. The local electric field is effective for the full charge of the oxide capacitance only under light radiation, and leads to an accurate method for ultra-thin insulators characterization excluding tunnel leakage current.