The Au/Pd/Ti-SiO2-GaAs structures with p- and n-GaAs have been investigated. The measurements of the capacitance-voltage characteristics at the different frequencies have been performed as well as the frequency dependence of MIS capacitance and conductance at fixed gate voltages. The complex frequency behaviour indicating the existence of a broad spectrum of the time constants has been observed. The equivalent circuit which allows to describe the obtained characteristics in a simple way has been presented. It contains two parallel branches including the constant phase elements (CPE1 and CPE2 in series with resistors R-1 and R-2) in parallel with the space charge layer capacitance, all of which is in series with an insulator capacitance and the series resistance. The parameters of that circuit have been estimated at different gate voltages as well as the characteristic time constants which describe the electron processes resulting in the observed frequency behaviour of the admittance characteristics. The CPE1 with R-1 are attributed to the electron processes evoked by GaAS-SiO2 interface states while CPE2 with R-2 describe the contribution of the localized electron states in the surface space charge region, which have been observed only in p-GaAs structures. (C) 2004 Elsevier B.V. All rights reserved.