The mechanism of the decomposition of a bronchodilator, S-nitroso-N-acetyl-D,L-penicillamine (SNAP) by a bronchoconstrictor, aqueous sulfite, has been investigated in detail. The decomposition was studied using a conventional spectrophotometer at 336 nm over the ranges: 0.010 ≤ $[S^{IV}]_T$ ≤ 0.045 mol $dm^{-3}$, 3.96 ≤ pH ≤ 6.80 and 15.0 ≤ θ≤ 30.0 ℃, 0.60 ≤ I ≤ 1.00 mol $dm^{-3}$, and at ionic strength 1.00 mol $dm^{-3}$ (NaCl). The rate of reaction is dependent on the total sulfite concentration and pH in a complex manner, i.e., $k_{obs};=;k_1K_2[S^{IV}]_T/ ([H^+];+;K_2)$. At 25.0 ℃, the second order rate constant, $k_1$, was determined as $12.5;{pm};0.15;mol^{-1};dm^3;s^{-1}$. ${Delta}H^{neq};=;+32;{pm};3 kJ;mol^{-1};and;{Delta}S^{ eq};=;-138;{pm};13;J;mol^{-1}K^{-1}$. The N-nitrosohydroxylamine-N-sulfonate ion was detected as an intermediate before the formation of any of the by-products, namely, N-acetyl-D,L-penicillamine. The effect of concentration of aqueous copper(Ⅱ) ions on this reaction was also examined at pH 4.75, but there was no dependence on $[Cu^{2+}]$. In addition, the $pK_a$ of SNAP was determined as 3.51 ± 0.06 at 25.4 ℃ [I = 1.0 mol $dm^{-3}$ (NaCl)].