An electrochemical quartz crystal impedance system has been applied to monitorgeneration of precipitate and adsorption of the precipitate onto An electrode during electrochmicaloxidation and Fe(CN)_6,3-/Fe(CN),6 4- electrochemical catalytic oxidation of L-cysteine in phosphateaqueous buffer (pH 7.4). Significant decreases in the resonant frequency and increases in themotional resistance and the static capacitance were found during the precipitate adsorption, and thefrequency shift found in solution was ca. 2 times that in air. Similar responses of quartz crystalimpedance and a white precipitate were obtained during redox titration of L-cysteine solutionusing K_3Fe(CN)_6 solution. FT-IR analysis indicated that the white precipitate is cystine. Theelectrode collection efficiency of the electrogenerated cystine was estimated. An electrochemical quartz crystal impedance system has been applied to monitor generation of precipitate and adsorption of the precipit onto an electrode during electrochmical oxidation and Fe (CN) 6,3 / Fe (CN), 6 4-Electro-chemical oxidation of L-cysteine ​​in phosphate aqueous Significant decreases in the resonant frequency and increases in themotional resistance and the static capacitance were found during the precipitate adsorption, and the frequency shift found in solution was ca. 2 times that in air. Similar responses of quartz crystalimpedance and a white precipitate were obtained during redox titration of L-cysteine ​​solutionusing K_3Fe (CN) _6 solution. FT-IR analysis indicated that the white precipitate is cystine. Theelectrode collection efficiency of the electrogenerated cystine was estimated.