Transfer of the nitroso group from nitrosothiols to thiols occurs very readily in aqueous solution particularly at pH > ≈8. The results are consistent with attack by the thiolate anion at the nitroso nitrogen atom of the nitrosothiol. Results have been obtained for the reaction of S-nitroso-N-acetylpenicillamine (SNAP) with thioglycolic acid and also for the reaction of S-nitrosocysteine (SNCys) with thiomalic acid. Both reactions showed the same kinetic characteristics. The results are discussed in terms of transnitrosation reactions of nitroso compounds generally, and also in the case of nitrosothiols, in terms of possible in vivo transnitrosation and subsequent decomposition of a possibly more unstable nitrosothiol to yield nitric oxide; this may have implications for the mechanism of action of nitric oxide in a range of physiological processes.