Abstract: Pathogens in sunlit surface waters can be damaged directly by UVB light. Indirect inactivation by reactive oxygen species (ROS) generated by sunlight interacting with external sensitizer molecules may also be important, but this mechanism has not been conclusively demonstrated. To better understand the role of ROS, we investigated the inactivation of MS2 coliphage, a commonly used surrogate for human enteric viruses, in water samples irradiated with a solar simulator and containing different types of sensitizers: waste stabilization pond (WSP) constituents, Fluka humic acid (FHA), and Suwannee River humic acid (SRHA). Inactivation of MS2 by the indirect mechanism was significant for all three sensitizers, and the efficiency of the sensitizers at inactivating MS2 was FHA > SRHA > WSP. Both dissolved and particulate fractions in the WSP water contributed to inactivation. In the WSP water, the indirect process was quantitatively more important than direct damage by UVB light, due to the rapid attenuation of UVB compared to the longer wavelengths that may initiate the indirect mechanism. Singlet oxygen (1O2) was the most important ROS involved in the inactivation of MS2. The addition of histidine, a 1O2 quencher, decreased inactivation, whereas inactivation rate constants increased in solutions of D2O. Selective quenchers for other ROS showed little or no protective effect. Inactivation in WSP water was a function of the steady-state 1O2 concentration and could be described by a second-order rate expression.