Abstract: The redox reaction of 2,2,6, 6-tetramethyl-4-hydroxypiperidine-1-oxyl radical with dl-cysteine in buffer solution of different pHhas been studied at room temperature under free of oxygen in the dark.It was found that in acidic buffer the oxidation product of cysteine was cysteic acid, in neutral buffer the product was cystine accompanied by a small amount of cysteic acid, whereas in basic buffer the product was cystine only. The reduction product of the nitroxide was identified as the corresponding hydroxylamine in all these cases.