The kinetics and mechanism of the reaction between hydroxylamine and chlorine in 0.1-1 M hydrochloric acid have been determined. In 10-fold or greater excess of NH3OH+, on the stopped-flow timescale, three distinct steps are observed : the initial rapid formation of an intermediate, which reacts with further hydroxylamine to form nitrous acid, HONO, followed by a slower reduction of HONO with excess NH3OH+ to form nitrous oxide. In the presence of a large excess of NH3OH+, the stoichiometry several minutes after mixing is Delta(NH3OH+)/Delta(Cl-2) = 1.1 +/- 0.2, which corresponds to a nearly quantitative formation of N2O. When the reactants are mixed in equimolar ratios, the principal nitrogenous product is nitrate. On the basis of the kinetics and stoichiometry, and by analogy with the well-established F3NO, the intermediate proposed is Cl3NO. This intermediate will hydrolyze rapidly to give NO3-, but in the presence of excess NH2OH it is reduced even more rapidly to give HONO, The proposed reaction sequence in a large excess of NH3OH+ corresponds to the following oxidation state changes : N(-I) --> N(V) --> N(III) --> N(I). The initial oxidation by 3 equiv of Cl-2 is consistent with a series of Cl+-transfer steps, where high acidity suppresses the rate of the first reaction with NH2OH but does not suppress the reactivity of the less basic transients, CINHOH and Cl2NOH.