Precursor ion scans using electrospray ionization-triple quadrupole mass spectrometry with or without ultra performance liquid chromatography preseparation were employed to examine the effects of enhanced/conventional coagulation on the formation of polar halogenated disinfection byproducts (DBPs) during chlorination. It is of interest that many new polar halogenated DBPs were generated as a consequence of enhanced or conventional coagulation, and a number of them were nitrogenous DBPs and/or had m/z values larger than 250. Some of the new polar halogenated DBPs showed up only in samples with enhanced coagulation but not in samples with conventional coagulation, although enhanced coagulation was better for controlling the overall DBP formation. The results suggest that the fraction of natural organic matter resistant to be removed by coagulation should have a low reaction potential with halogens; after enhanced or conventional coagulation, halogens are forced to react with the remaining fraction of natural organic matter, producing new polar (nitrogenous) halogenated DBPs. In addition, enhanced coagulation altered the speciation of halogenated DBPs by elevating the ratio of bromine-containing to chlorine-containing DBPs; this phenomenon was observed for both polar and apolar halogenated DBPs, but polar DBPs were more susceptible to be altered. (C) 2010 Elsevier B.V. All rights reserved.