Isoxaflutole (IXF; BalanceTM) belongs to a new class of isoxazole herbicides. Isoxaflutole has a very short half-life and rapidly degrades to a stable and phytotoxic metabolite, diketonitrile (DKN). Further degradation of DKN produces a nonbiologically active benzoic acid (BA) metabolite. DKN was previously discovered to rapidly react with hypochlorite (OCl-) in tap water, yielding the benzioic acid metabolite as the major end product. Stoichiometry of the reaction suggests a molar ratio of OCl-/DKN near 3 (2.95 ± 0.36). During the analyses for elucidation of the degradation pathway, another stable chlorinated byproduct was identified when OCl- was depleted. This chlorinated compound was isolated and further characterized by its ion fragmentation patterns and isotope profiling. The proposed chemical structure was consistent with the structure predicted by mass spectrometry simulation software. Based on the stoichiometry of the reaction and structure of the reaction products, a two-step reaction via nucleophilic attack of the keto carbons by OCl- reaction mechanism was proposed. Due to the high stability and mobility (hydrophobicity between DKN and BA) of the chlorinated byproduct identified from this work, it has potential to be an environmental contaminant in agronomic settings if chlorinated tap water is used to prepare solutions for field spraying.