TheS-nitrosothiols 2-acetamido-2-deoxy-S-nitroso-1-thio-&bgr;-D-glucopyranose 3,4,6-triacetate (GPSNO) andS-nitroso-N-carbamyl-D,L-penicillamine (SNCP) were synthesized byS-nitrosation of the corresponding thiols, isolated, and fully characterized. The nitrosothiol (TGSNO) from 1-thioglycerol was obtained as a red gelatinous liquid, which decomposed rapidly at room temperature and so was not characterized. The kinetics of decomposition of GPSNO showed that there is a surprisingly large thermal pathway overlaid with a Cu2+/RS-catalyzed reaction. The results strongly suggest that the product disulfide complexes Cu2+(for which there is some spectral evidence), leading to incomplete conversion by that route. Ascorbate also acts as a Cu2+reductant. AnotherS-nitroso sugar,S-nitroso-1-thio-&bgr;-D-glucose (SNTG), behaved very similarly from solutions generated and used in situ. The decomposition of TGSNO shows induction periods suggesting that slow initial generation of Cu+(the true catalyst) is taking place. There appears to be also a significant alternative pathway (analogous to that found for GPSNO), where the rate appears to be independent of [Cu2+], but very unusually this pathway is effectively halted by addition of EDTA either at the start of the reaction or at a later time. Reaction schemes are put forward to account for these unusual reaction characteristics.Key words:S-nitrosothiols, nitric oxide, ascorbate, copper catalysis.