Abstract: The process of adsorption of lysozyme onto bare and thioglycolic acid or n-dodecanethiol modified Au electrodes in a phosphate buffer (pH=7.3) solution has been investigated in situ using an electrochemical quartz crystal impedance system(EQCIS). Equivalent circuit parameters and resonance frequencies in piezoelectric quartz crystal(PQC) resonance as well as the electrical double layer capacitances were obtained and analyzed. A scheme of two consecutive reactions occurring at the interface was used to analyze the adsorption kinetics, suggesting that the double layer capacitance is more sensitive to the rearrangement of the adsorbed protein molecules compared with the resonance frequency. The effect of potential and hydrophilicity of electrode surface on the lysozyme adsorption was investigated. It is found that a more negative potential resulted in a larger responses of PQC resonance and the amount of adsorbed lysozyme on a hydrophobic surface was larger than that on a hydrophilic one. The standard heterogeneous rate constants of ferricyanide/ferrocyanide and charge transfer resistance before and after the lysozyme adsorption were determined. Decrease in standard heterogeneous rate constants of ferricyanide/ferrocyanide and increase in charge transfer resonance were observed after the lysozyme adsorption, showing that the reversibility of ferricyanide/ferrocyanide redox reaction was decreased.