This study investigated whether colloid-sized particles can enmesh and protect viruses from 254-nm ultraviolet (UV) light and sought to determine the particle characteristics (e.g. size, chemical composition) that are most relevant in causing a protective effect. Two viral surrogates (MS2 coliphage and bacteriophage T4), three types of particles (kaolin clay, humic acid powder, and activated sludge), two coagulants (alum and ferric chloride), two filtration conditions (none and 0.45 microm), and two UV doses (40 and 80 mJ/cm2 for MS2 coliphage; 2 and 7 mJ/cm2 for bacteriophage T4) were considered in a series of bench-scale UV collimated beam experiments. Transmission electron microscopy was used to qualitatively confirm the phage particle-association after coagulation. Humic acid and activated sludge floc particles shielded both viral surrogates to a statistically significant degree (with >99% confidence) relative to particle-free control conditions, while the kaolin clay particles provided no significant protection. The results of the study suggest that particles <2 microm in diameter are large enough to protect viruses from UV light and that particulate chemical composition (e.g. UV-absorbing organic content) may be a critical factor in the survival of particle-associated viruses during UV disinfection.