Careful analysis of d.c. polarographic mean and instantaneous currents helps to explain why the anodic electrolytic reaction of cysteine on mercury electrodes proceeds differently in acetate and in berate buffer solutions, even though in both solutions cysteine as well as the product of its electrode reaction are adsorbed at the electrode. In acetate buffer of pH 4.7, cysteine is adsorbed through the interaction of its sulphur atom with mercury, which ultimately leads to a steeper than reversible slope of the first polarographic wave, to a transformation of that wave into an adsorption pseudo-prewave, and to a ready appearance of the second wave. The adsorption of cysteine from berate buffer of pH 9.4, in contrast, causes a marked increase of the electrode capacity but only little interference in the electrode process.