AbstractHydroxylamine adds reversibly to C‐6 of uracil basesviaa multistep mechanism. Equilibrium constants measured at severalpH values, 25°, μ = 3.0Mare: uracil, 0.25M−1; 5‐iodouracil, 0.30M−1, 5‐bromouracil, 0.24M−1, 5‐chlorouracil, 0.06M−1and 5‐fluorouracil, 5.11M−1. The kinetics of hydroxylamine addition to both 5‐bromo‐and 5‐iodouracil are complex. At low hydroxylamine buffer concentrations, the rate constants are second‐order with respect to hydroxylamine buffer but at higher concentration a first‐order dependence is approached. Hydroxylamine elimination from 5‐iodo‐6‐hydroxylamino‐5,6‐dihydrouracil is subject to general base catalysis by tris(hydroxy‐methyl)aminomethane but at higher concentrations the rate constants are not proportional to the concentration of general base. This reaction is subject to solvent effects where increasing ethanol concentration depresses the rate when measured at constantpH. These kinetics can be rationalized in terms of a multistep reaction pathway in which hydroxylamine addition to C‐6 of the halogenated uracil precedes general acid catalyzed proton transfer to C‐5 yielding the