This study evaluates the synergistic effects of UV coupled with secondary disinfectants using a model distribution system. In particular, the research employs eight annular reactors (ARs), which have been widely used in the water industry for evaluating biofilm growth under conditions representative of drinking water distribution systems. The chemical disinfectants that are being evaluated for residual disinfection are chlorine dioxide, monochloramine, and free chlorine. Results indicate that there are synergistic effects between UV and the chemical disinfectants for controlling microbiological regrowth in drinking water distribution systems. In particular, in the reactors containing UV treated water both chlorine dioxide (ClO2) and free chlorine (Cl2) performed better at reducing the suspended heterotrophic bacteria. When operated with either a low or high disinfection residual the ClO2 reactor (UV treated) resulted in a greater than 2-log reduction in heterotrophic plate counts (HPCs) over the reactor that was not treated with UV light. The Cl2 reactor (UV treated) resulted in approximately 2-log reduction of HPCs over the reactor that was not treated with UV light. In terms of biofilm reduction, UV disinfection resulted in a reduction of approximately 0.5-log difference for the ClO2 and NH2Cl ARs. No measurable difference of UV disinfection was detected for the AR receiving free chlorine. ARs receiving no chemical disinfection resulted in similar biofilm and suspended HPC levels regardless of whether UV disinfection was present or not. Overall, these findings indicate that UV prior to chemical disinfection enhances microbial control in distribution systems. Practically, these results would indicate that a dual-barrier disinfection system should be considered for pathogen protection at the plant and distribution system. Includes 10 references, tables.