To estimate the amount of reactive dissolved organic matter (DOM) that produces polybromomethanes in coastal surface seawater, DOM was brominated by algal bromoperoxidase (BrPO) in the presence of H₂O ₂ and bromide. Bromoform (CHBr₃) was the predominant species produced year-round in different waters. Production from nearshore seawater was greater than that from offshore seawater. The elevated CHBr₃ production in the spring was associated with increased biological productivity and the resulting DOM. A large CHBr₃ production potential (∼35 Gmol) was found in coastal waters independent of in vivo algal production. Terrestrially-derived DOM was responsible for dibromomethane (CH₂Br₂) production as its highest production was associated with the highest precipitation and presumably terrestrial runoff. Higher MW DOM fractions had more reactive DOM, as supported by higher CHBr₃ production rate: [DOC] ratios. My results suggest that when cell-free/cell-attached BrPO is present in seawater, DOM could be brominated by BrPO-generated HOBr to produce CHBr₃ and other volatile polybromomethanes.