A new technique for determination of the flat-band potential, V-fb, based on ac resistance measurements is presented. It is shown that impedance measurements, performed with and without illumination at a single, relatively high frequency of 75 kHz can yield information about the semiconductor space charge layer and the position of the energy levels of a semiconductor/electrolyte system. The method was successfully tested with n-type and p-type Si samples of different resistivities, used as working electrodes in HF containing electrolyte solutions. The results obtained were compared with the flat-band potential data for silicon as known from the literature.