The kinetics of the reaction between nitrous acid and sulphamic acid have been studied at three temperatures over the acidity range pH 5 to 3M-[H+]. At acidities lower than 0·248M-[H+], reaction only occurs by the attack of the nitrous acidium ion on sulphamate ion. Attempts to catalyse other nitrosation pathways failed. This exclusive, although low, reactivity of the suphamate ion for the nitrous acidium ion is discussed. Increase of acidity to 3M-[H+] results in the introduction of another reaction pathway involving sulphamic acid. The deuterium solvent isotope effect is discussed in terms of mechanism.