The differential capacitance Of SiO2 ultra-thin layers on Si substrate is greatly sensitive to the space charge generated within the semiconductor. In the potential scan, 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 semiconductor/oxide/electrolyte (SOE) structure, under zero current flow. Depending on the bias potential we obtained RC equivalent circuits corresponding either to the depletion layer or to the oxide dielectric film. A novel aspect of the work is that both R and C components were derived from the data processing. In a previous work the investigation was focussed on the depletion layer, and lead to values of the resistance term in the range of a few kOmega to a few MOmega cm(2), while the capacitance value was a few 10(-2) muF cm(-2). These results were consistent with a theoretical treatment of the bias voltage dependence of the charge distribution near the flatband potential, and constitute a new technique for the determination of the fb potential versus a reference electrode. The present work is devoted to the electrical properties of the Si surface oxide layer. The leakage resistance term of the thermal oxide layer, a few nm thick, was found equal to several 10(8) Omega cm(2). But, the electric field within the semiconductor is not effective for the full charge of the oxide capacitance even when the polarization creates an accumulation layer. In accordance with the computed electric field within the semiconductor, the right value of the capacitance can be reached easily when the wafer is submitted to light radiation and provided the polarization of the substrate is such as to generate an inversion layer. This property leads to an accurate method for ultra-thin insulators characterization excluding tunnel leakage current. (C) 2003 Elsevier B.V. All rights reserved.