In the treatment of surface water for the preparation of municipal drinking water, chlorine-containing disinfectants can react with natural organic matter (NOM) and source water bromide to form harmful disinfection byproducts (DBPs). Trihalomethanes (THMs) and haloacetic acids (HAAs) are currently the only carcinogenic organic DBPs regulated in tap water by the United States Environmental Protection Agency. While these DBPs are important to monitor, unregulated MX (3-Chloro-4-(dichloromethyl)-5- hydroxy-2(5H)-furanone) and its brominated analogues present a larger potential health risk than HAAs and THMs due to their enhanced mutagenicity. Recently, halogenated furanones were included in a DBP survey of U.S. drinking water treatment plants, where they followed similar formation and removal trends with THMs and HAAs. Small ketone components of fulvic acid have been proposed as common precursors to THMs and di- and trihalogenated HAAs. This hypothesis is supported by concurrent formation of THMs and HAAs observed after chlorination of NOM fractions or model compounds. On the other hand 3,5-dihydroxybenzaldehyde has been cited as the simplest model of the base structure for humic substances, and some researchers have focused on the significance of the aldehyde group in the meta position to two hydroxyl groups on the benzene ring in the formation of MX-analogues via a THM and HAA formation pathway. Clearly, much remains to be learned about the mechanisms of formation of these 3 groups of DBPs. In this study, the mechanisms of formation for the THMs, HAAs, and halogenated furanones (MX-analogues) were investigated using Uniform Formation Conditions (UFC) for chlorination of three different, well-characterized natural organic matter (NOM) isolates. UFC chlorination was chosen to simulate conditions in drinking water treatment plants and to target the DBP precursors encountered under these conditions, with and without the addition of bromide. Includes 15 references, figures.