The rate of reduction of nitrite by trimethylamine-borane was followed by observing the decrease in nitrite absorbance under pseudo-first-order conditions. The reaction is acid-catalyzed and exhibits a first-order dependence on both amine-borane and total nitrite concentration. The molar equivalence of NaNO2 to (CH3)(3)NBH3 = 2:1. Equimolar amounts of hydrogen and nitrous oxide are formed, and the molar ratio of nitrite reacted to N2O produced is 2:1. In concentrated HCl or H2SO4, a correlation of rate with the Hammett acidity function, h(0), is observed. The reaction is subject to a pronounced inverse solvent isotope effect (k(D2O)/k(H2O) similar or equal to 2.7) and a modest normal substrate effect (k((CH3)3N . BH3)/k((CH3)3N . BD3) similar or equal to 1.4). The reaction is first-order in H3O+ in the region pH 0.7-2.7, but a second-order dependence is observed above pH 4 with the transition occurring at pH similar or equal to pK(a) for HNO2. Results are consistent with a mechanistic model involving preequilibration protonation of molecular nitrous acid followed by-rate-limiting hydride attack on H2ONO+ or free NO+ to produce nitrosyl hydride as a reactive intermediate.