In strongly aqueous acid solution (±)-2-acetylamina-2-carboxy-l,1-dimethylethyl thionitrite (RSNO) undergoes denitrosation to give the corresponding thiol. The reaction is reversible and normally lies well over on the side of the thionitrite, but can proceed in the denitrosation direction in the presence of traps for free nitrous acid such as added sodium azide or sulphamic acid. At sufficiently high trap concentration to ensure complete irreversibility, the reaction was found to be first-order in RSNO and acid-catalysed; the observed reaction rate constant is approximately proportional to hO. Catalysis was observed by Cl–, Br–, SCN–, and SC(NH2)2 and the second-order rate constants were 4.6 × 10–3, 13.0 × 10–3, 34 × 10–3, and 27 × 10–3 l mol–1 s–1, respectively. The order of reactivity is as expected with the exception of that of thiourea. N-Methyl-4-nitroaniline was converted quantitatively to the corresponding nitrosamine by RSNO, but in the presence of added sodium azide, no nitrosamine was detected, this shows that RSNO like nitrosamines generally and alkyl nitrites, acts as a nitrosating agent towards amines etc., under these experimental conditions, by prior hydrolysis or via nitrosyl halide etc. formation. A kinetic analysis has enabled the reactivity of the nitrous acid traps hydrazoic acid and sulphamic acid to he established quantitatively.