The kinetics of decompostion of sodium trioxodinitrate. Na₂[N₂O₃], have been measured over the range pH 1–10 at several temperatures. At pH > 4, the rate-determining step is breakdown of [HN₂O₃]^– to [NO₂]^– and N₂O. The pK for [HN₂O₃]^– at I= 0.25 mol dm^–3 and 25 °C is 9.35, and λ_max.= 237nm. At lower pH values, the rate increases with increasing acdity with production of NO, which is the predominant product at pH2. At these pH values k_obs, the measured first-order rate constant, increases with increasing concentration of Na₂[N₂O₃]. This is attributed to a reaction with trace amounts of HNO₂, giving NO and reforming [NO₂]^–, rather than to an enhanced instability of H₂N₂O₃. Extrapolation of k_obs. to zero [N₂O₃^2–] at pH < 3 gives k_obs.* which corrsponds to decomposition via[HN₂O₃]^–; k_obs.* decreases with decreasing pH. showing that H₂N₂O₃ is stable compared to [HN₂O₃]^– in the absence of nitrite and allowing the estimation of pK₁ca.3·0. Addition of [NO₂]^– to Na₂[N₂O₃] at pH 5 results in the production of NO, the use of [¹⁵NO₂]^– showing that this is not attributable to the disproportionation of HNO₂ and also that both molecules of NO produced in the reaction are derived from the nitrogen atoms of [N₂O₃]^2–. The acid-catalysed HNO₂-catalysed reaction at lower pH obeys the rate equation Rate ∝[H⁺][HNO₂][H₂N₂O₃], but the value of the third-order rate constant is too high for a diffusion-controlled electrophilic nitrosation reaction. Added ethanol at these pH values has very marked inhibitory effect on the rate, so it is suggested that formation of NO results from a free-radical chain reaction.