This study investigated the influence of virus-particle association on ultraviolet (UV) disinfection. Specifically, the objectives of the study were to determine whether colloidal particles could protect viruses from UV light and to determine which particle characteristics enhance protection. Three types of particles (kaolin clay, humic acid, and activated sludge particles), two coagulants (alum and ferric chloride), two viral surrogates (MS2 coliphage and bacteriophage T4), two filtration conditions (no filtration or 0.5- micron filtration), and two UV doses were considered in a series of jar tests designed as factorial experiments to evaluate the effect of particle characteristics (size, composition) on UV disinfection of particle-associated viruses. The results suggest that particulate organic content is a critical parameter in the protection of waterborne viruses from UV disinfection. The findings reinforce the importance of a multi-barrier approach to pathogen control that includes the effective physical removal of particles and particle-associated pathogens prior to disinfection. Includes 32 references, figures.