Hypothiocyanite (OSCN-) hydrolyzes under alkaline conditions to give thiocarbamate-S-oxide (H2NC (= O)SO-, the conjugate base of carbamothioperoxoic acid) via a mechanism that involves rate-limiting nucleophilic attack of 0H(-) on OSCN-, followed by fast protonation (with no net consumption of H+/OH- at pH 11.7). Thiocarbamate-S-oxide has been characterized by C-13 NMR, N-15 NMR, UV spectroscopy, and ion chromatography. It has also been independently synthesized by the reaction of thiocarbamate (H2NC(= O)S-) and hypochlorite (OCl-). The properties of thiocarbamate-S-oxide that is produced by hydrolysis of OSCN- and by oxidation Of H2NC(= O)S- are the same. The possible relevance of thiocarbamate-S-oxide in human peroxidase defense mechanisms remains to be explored.