The preparation and some of the properties of the disulfoxide of I-cystine have been described in a previous paper (1). Despite its intermediate level of oxidation, which is suggestive of chemical instability, the disulfoxide was shown to be relatively stable, especially in aqueous acid solution, a property which was utilized in its separation from other products of non-aqueous oxidation. However, further study revealed that all aqueous solutions of the disulfoxide undergo dismutative decomposition, or hydrolytic oxidation and reduction, which results in the formation of cystine and acid derivatives. The rate of this decomposition increases with increasing pH (and temperature) ; in N HCI 15 days are required for completion of the reaction, while slightly more than 15 minutes are sufficient at pH 8. Apparently the acidity of the solution also affects the course of the reaction (possibly through an isomerism of the disulfoxide), since analytical evidence indicates the acid derivative formed by alkaline decomposition to be the sulfinic acid (R-S(O)-OH), while the evidence from acid decomposition points to the formation of several acids. The sulfinic acid has been isolated and identified; it possesses some differences from that described by Schubert (2). The chemical reactivity of the disulfoxide is illustrated by its rapid reaction with thiol compounds and NaCN. The sulfinic acid seems to be a product common to all these reactions.