Humics are a substantial component of the total organic carbon content of water, and as such, present problems during disinfection due to their contribution to disinfectant demand and disinfection byproduct formation. Additionally, humics and their oxidized byproducts formed during disinfection have been implicated in the production of bacterial growth in distribution systems, including biofilm formation. While numerous researchers have demonstrated that natural organic matter (NOM)/humic substances oxidized by ozone can be readily used as a carbon source by heterotrophic bacteria, limited research is available on the effect of varying doses of UV light. There is also very little information on the changes in the characteristics of NOM caused by UV light. The goal of this project was to investigate the use of ultraviolet light for disinfection of drinking water. The work plan included two objectives: determining whether water containing natural organic matter that is treated with UV light is more likely to support microbial growth on pipe surfaces; and, assessing the change in the nature of humic substances exposed to UV light. For the first objective, small model distribution systems were fed humic substances that were UV treated, and the amount of biofilm and released suspended cells compared with systems fed water that has not been treated with UV light. For the second objective, methods used to assess the manner in which other oxidants changed the composition of a standard humic substance solution were applied to determine the potential impacts of UV treatment. Includes 4 references.