The Au/Pd/Ti-SiO2-GaAs structures with PECVD deposited insulator layers have been investigated. The (100) oriented n-GaAs wafers with and without (NH4)(2)S surface treatments have been used. The measurements of capacitance-voltage characteristics at different frequencies as well as the frequency dependence of MIS capacitance and conductance at fixed gate voltages have been performed. The large frequency dispersion of MIS admittance has been observed. The model of insulator-semiconductor interface as disordered system with localized electron states distributed in energy and in space offers the possibility of an explanation of the experimentally observed broad spectrum of time constant. It has been stated that sulfur treatment of GaAs surface introduces the negative fixed charge at SiO2-GaAs interface rather than reduces the density of interface states.