The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5-8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl -, NH + 4 , and N 2 O. The rate law is given byAt an ionic strength of 1.2 M, at 25 • C, and under pseudo-first-order conditions, k + = (1.03 ± 0.06) ×10 3 L · mol −1 · s −1 and k 0 = 91 ± 15 L · mol −1 · s −1 . Isotopic studies demonstrate that both nitrogen atoms in the N 2 O come from the NH 2 OH/NH 3 OH + . Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be H ‡ = 36 ± 3 kJ · mol -1 and S ‡ = −66 ± 9 J · K −1 · mol −1 , and H ‡ = 12 ± 2 kJ · mol −1 and S ‡ = −168 ± 6 J · K −1 · mol −1 , respectively, and confirm that the transition states are significantly different for the two reaction pathways. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: [124][125][126][127][128][129][130][131][132][133][134][135] 2006