The U.S. Environmental Protection Agency (EPA) is considering changes to the disinfectant and disinfection byproducts (D/DBP) rule. Specifically, there may be a shift from the currently regulated five haloacetic acids (HAA5) to nine (HAA9), which would include four additional brominated compounds. Therefore, there is a need to better understand the toxicology, public health, treatment, and compliance implications of a shift from an HAA5- to HAA9-based regulation. This project will increase understanding of these implications by (1) assessing the potential impacts of new regulatory levels for HAA5, the brominated HAAs, or HAA9; (2) developing a database and analysis that will be made available to water systems for discussion with regulatory authorities; (3) evaluating the weight of evidence regarding the relative toxicity of HAA9 compounds; (4) building an understanding of the benefits of compliance technologies for a future rule; (5) providing guidance on the consequences of implementing changes to respond to a revised maximum contaminant level for HAAs; (6) and developing materials to support clear, concise communication about the issues surrounding a move from an HAA5- to HAA9-based regulation. Research partner: American Water Works Association.