The zero current impedance of a silicon substrate in a semiconductor/oxide/electrolyte structure was used to identify the contribution of the depletion layer under various bias potentials. Careful measurements using p-Si in a HCl solution within the potential range of 0 to -1 Vvs. a saturated calomel electrode (SCE) led to the determination of the corresponding equivalent circuits as a function of the bias potential. Modeling the circuit as a constant phase element proved that the imaginary component was a pure capacitor C-SC in parallel with a pure resistance R-SC. Experimental data showed that these two components undergo a steep variation when the system approaches the silicon flatband potential situation. A novel fundamental development is presented, assuming that the gradient of potential inside the material is small enough for a simplified treatment based on the linearization of the exponential function. The steep increase in the vicinity of the flatband potential of the space charge capacitance and conductance was confirmed. This constitutes a useful tool for electrochemical studies to determine the flatband potential and band curvature as a function of the sample potential measured vs. the SCE reference electrode. (C) 2002 The Electrochemical Society.