The reaction of cystine with sodium sulfide in 0.2M sodium hydroxide solution produces a product with an absorption maximum at 335 m/i; it is postulated that this absorption is due to (-SSCH2CHNH2COO-). In the presence of excess sulfide this species reacts further, and, overall, there are produced two cysteinate ions and disulfide or polysulfide ions. When the reaction mixture is made strongly acid, the absorption at 335 µ disappears, sulfur precipitates, and cysteine is found; with 5-10 equivalents of sodium sulfide the reaction of cystine is more than 95% complete. The change of optical rotation with time in mixtures of cystine and sulfide has been measured, and the approximate speed of reaction determined.Cystine is a substance of considerable importance in biochemistry, primarily because of its participation in the composition of many proteins, and of the unique role which it plays in establishing and maintaining protein structure. Owing to the interest which attaches to this substance, many of its physical and chemical properties have been intensively studied, but these studies have not extended, except in a cursory way, to the reaction with sodium sulfide.1•2 This is rather surprising, because the solubilizing and softening action of sodium sulfide on wool and hair, a phenomenon of some practical importance, is largely due to reaction with the cystinyl residues in these protein materials.3 Sodium sulfide also interacts in an interesting way with globular proteins, for instance with the enzyme papain.4 The present investigation was undertaken to obtain fundamental information concerning this type of reaction.Before entering upon a discussion of the experimental results, it is necessary to make reference to two problems which are pertinent to that discussion, namely, the acid-base properties of sulfide ion, and the nature and absorption of polysulfide ions.The second ionization constant of hydrogen sulfide is so small that sulfide ion is almost completely hydrolyzed in aqueous solution; unfortunately, there is lack of agreement in the values reported for the ionization constant,5•6 so that the relative concentrations of sulfide and hydrosulfide cannot be calculated accurately. In most of the experiments to be described, 0.2M sodium hydroxide ivas used as a medium. This would serve to reduce the hydrolysis, and maintain a nearly constant, although undetermined, ratio between the sulfide and hydrosulfide ions. Even in this alka-(1)