The capacitance and the conductance vs. the frequency characteristics of Au/Pd/Ti-SiO2-(n) GaAs structures with and without (NH4)(2)S-x treated GaAs surface have been investigated. For normalized conductance G(m)/omega characteristics, the peaks with the position dependent on the gate voltage have been observed for both types of structures and also the peaks with the position not dependent on the bias for the (NH4)(2)S-x treated samples. That behaviour has been attributed to the interface state contribution and the deep trap contribution, respectively. The equivalent circuit has been presented which allows for the description of the observed admittance peculiarities in a simple way It contains: the insulator capacitance, the series resistance, the space charge capacitance and the two parallel branches including the constant phase elements (CPE1 and CPE2) connected in series with resistances. The values of parameters of the equivalent circuit elements at different gate voltages have been determined. These elements are attributed to the physical phenomena in different regions of the analysed system. The CPE1 (with parameter ni similar to0.54) connected in series with resistance describes the electron processes in the SiO2-GaAs region leading to the frequency dispersion of the electrical characteristics of the investigated structures in the frequency range which depends on the bias, while CPE2 (with parameter n, -0.89-0.95) connected in series with resistance models the deep traps admittance. (C) 2003 Elsevier B.V. All rights reserved.