Measurements of the capacitance and conductance of Au/Pd/Ti-SiO2-GaAs structures with different treatments of GaAs surface as a function of frequency (100 Hz-1.6 MHz) at fixed gate voltages have been performed and large dispersion has been observed. The analysis of these characteristics using the impedance spectroscopy method has allowed for the construction of the electrical equivalent circuit for the investigated structures which contains: the insulator capacitance, the series resistance, the space charge capacitance and the constant phase element (CPE) connected in series with resistance. The values of the parameters of the circuit elements have been determined by the broad range of the gate voltage. These elements are attributed to the physical phenomena in different regions of the analysed systems. The CPE (with parameter nsimilar to0.53-0.57) connected in series with resistance describes the complex electron processes in the GaAs-SiO2 region resulting in the large frequency dispersion of electrical characteristics of the investigated structures.