In this study, the effect of bromide ion on monochloramine loss was examined in the presence of natural organic matter (NOM). Bromide ion catalyzes monochloramine autodecomposition by the formation of active bromine intermediates. However, in the presence of NOM, the active bromine may either react with NOM or monochloramine. In the presence of monochloramine and NOM, bromide increases monochloramine loss but not if bromide catalyzed autodecomposition and monochloramine reaction with the NOM are dominant loss pathways. Also, significant quantities of brominated haloacetic acids (HAAs) are formed. Therefore, resolution of the active bromine pathways is critical to the understanding of monochloramine loss and formation of brominated HAAs. This work shows the preliminary results of modeling monochloramine loss in the presence of NOM and bromide ion semi-mechanistically. The ability to resolve the monochloramine loss pathways in the presence of multiple reactive constituents, such as NOM and bromide, will provide a relationship to model the formation of chlorinated and brominated HAAs. Includes 10 references, table, figures.