Disinfection byproduct (DBP) formation depends on the presence and reactivity of oxidants and precursors (e.g., natural organic matter [NOM], bromide, and/or iodide ions) and is influenced by water quality and treatment conditions. Controlling DBP formation has historically focused on removing NOM through various treatment processes, altering disinfectant types or sequences, or changing treatment conditions. Bromide and iodide ions react with oxidants and NOM to form intermediates and produce regulated and emerging organic or inorganic DBPs. This project investigated the occurrence of bromide and iodide in drinking water sources; determined their role in the formation of regulated and emerging DBPs; and developed a better understanding of how seasonal changes in watersheds, regional geochemistry, and other factors impact these key inorganic DBP precursors. Published in 2022.