Electrochemical quartz crystal impedance studies on L-cysteine oxidation in aqueous solutions were conducted. The precipitation of electrogenerated species on the Au electrode surface was found in acetic and phosphate buffers. Changes in the equivalent circuit parameters, resonant frequencies and half peak width of the conductance spectra (Delta f(G1/2)) Obtained simultaneously during electrochemical experiments were discussed. Significant changes in the motional resistance and Delta f(G1/2) revealed that the adhering precipitate acted as a non-rigid mass loading to the piezoelectric quartz crystal resonance. The resonant frequency shift found in solution was approximately twice that in air. Lots of loose and separate precipitate agglomerates formed and stood like islands on the electrode surface, and for a frequency shift of - 10 000 Hz in the acetic buffer, the height of the precipitate agglomerate could be greater than 5 mu m, as found by SEM. In addition, a white precipitate and similar responses of the equivalent circuit parameters were obtained during ferricyanide titration of L-cysteine in acetic, phosphate and ammonia buffers. IR analysis revealed that the white precipitate is cystine. Electrode collection efficiencies of cystine were evaluated. Moreover, the frequency-point-selection fitting routine has been discussed and taken for analyses of the admittance data, and an equation of Delta F-G1/2 as a function of the equivalent circuit parameters was given.