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 λmax.= 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 kobs, 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 kobs. to zero [N2O32–] at pH < 3 gives kobs.* which corrsponds to decomposition via[HN2O3]–; kobs.* 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.