The kinetics of nitrosation of cysteine, cysteine methyl ester, N-acetylcysteine, penicillamine, N-acetylpenicillamine, glutathione and thioglycolic acid was undertaken. These thiols exhibited identical rate laws which are interpreted as nitrosation at sulphur by H(_2)NO(_2)(^+)/NO(^+). The rate constants determined show the high reactivity of thiols towards the nitrosating agent. The nucleophile catalyzed reactions were also investigated and the order of reactivity NOCl > NOBr > NOSCN was observed. Normally in these nucleophile catalyzed reactions there is a first order dependence on [thiol]. However, for N-acetylcysteine and thioglycolic' acid at high [thiol] the rate of formation of NOX tends to become the rate determining stage. The difference in rate constants between cysteine and penicillamine and their N-acetyl derivatives is explained in terms of internal stabilization. The decomposition of S-nitrosocysteine (S-NOCys) at pH 5.5, 7 and 9.8 in the presence and absence of C1(^-), Br(^-) and SCN(^-), and also alanine and sodium bicarbonate at pH 7, and S-nitrosoglutathione (GS-NO) at pH 7 in the presence and absence of alanine, C1(^-), and sodium bicarbonate was studied. The decomposition profiles were complex, but showed that S-NOCys was least stable at pH 7, and that GS-NO was more stable than S-NOCys. The addition of the aforementioned species did not significantly affect the rate of decomposition of the thionitrites. Finally the potential of S-NOCys, GS-NO and S-nitroso-N-acetyl- penicillamine as nitrosating agents towards amines was investigated at pH 7 and pH 8. These thionitrites nitrosated morpholine to give approximately the same yield of N-nitrosomorpholine (ca -17%) at pH 7, and less at pH 8 for S-NOCys and GS-NO. The addition of sodium acetate, sodium chloride, sodium bicarbonate, alanine and glucose, compounds liable to be present in vivo, did not significantly affect the yield of N-nitrosomorpholine. The transnitrosation reaction was complete before total decomposition of the thionitrite and a direct reaction between the thionitrite and morpholine is proposed.