Hydroxylamine is oxidised by nitric acid to form dinitrogen monoxide and nitrous acid, the proportions varying with reaction conditions. The yield [HNO₂]∞/[NH₃OH⁺]₀ is a maximum at ca. 4–5 mol dm^–3 HNO₃, and is also a function of the hydroxylamine concentration. In 5 mol dm^–3 HNO₃ the limiting yield is ca. 0.85 at very low initial hydroxylamine concentrations, but decreases towards zero at higher values of [NH₃OH⁺]₀. Reaction is only observed at sufficiently high nitric acid concentrations; at 25°C the cut-off point is ca. 2.5 mol dm^–3 HNO₃. The reaction is characterised by an induction period, followed by a rapid autocatalytic process. Addition of nitrite eliminates the induction period, while addition of nitrite scavengers completely prevents any reaction. Nitrous acid is an essential catalyst for the reaction, and the initial rate of reaction obeys the equation d[HNO₂]/dt=V₀=K[HNO₂][NH₃OH⁺]. Isotopic experiments, using ¹⁵N-enriched hydroxylamine show that virtually all of the N₂O arises from reaction between HNO₂ and hydroxylamine. The mechanism suggested involves oxidation of unprotonated hydroxylamine by N₂O₄ to form the nitroxyl diradical HNO; this is then further oxidised to HNO₂, which reacts with hydroxylamine to form N₂O.