Pharmaceutical compounds represent a rapidly emerging class of environmental contaminants. Such compounds were recently classified by the U.S. Geological Survey, including several antibiotics. An LC-MS/MS screening method for the top five antibiotics in drinking water was developed and validated using a Shimadzu LC-MS-IT-TOF. The separation was performed using a Waters Acquity UPLC BEH C18 column with a gradient elution. Sulfamethazine was exposed to conditions intended to mimic drinking water chlorination, and samples were collected and quenched with excess sodium sulfite. Kinetics of sulfamethazine degradation was followed as well as the formation of the major chlorinated byproduct (m/z 313). For the screening method, all five antibiotic peaks were baseline resolved within 5 minutes. Additionally, precision and accuracy of the screening method were less than 15%. Degradation of sulfamethazine upon exposure to drinking water chlorination occurred by first order kinetics with a half-life of 5.3 × 10(4) min (approximately 37 days) with measurements starting 5 minutes after chlorination. Likewise, the formation of the major chlorinated product occurred by first order kinetics with a rate constant of 2.0 × 10(-2). The proposed identification of the chlorinated product was 4-amino-(5-chloro-4,6-dimethyl-2-pyrimidinyl)-benzenesulfonamide (C12H13N4O2SCl) using MS (n) spectra and databases searches of SciFinder and ChemSpider.