Photocatalytic inactivation tests were conducted on TiO2 suspensions containing bacteriophage MS2 under the irradiation from a black-light fluorescent lamp in the presence of glucose or ethanol as a model impurity. In the absence of impurity, the semi-logarithmic plot of relative phage titer decreased linearly obeying a first-order reaction, while the addition of glucose or ethanol gave plots with curved profiles of decreasing phage titer. These results suggest that the presence of organic impurity changes the kinetics of the photocatalytic phage inactivation. The profiles of phage titer during the photoreaction could be kinetically analyzed by applying a series-event model. Estimating the kinetic parameters (lethal number, n, and apparent inactivation rate constant, k') obtained from the applied model, it was found that an increase in the concentration of organic impurity resulted in an increase in n value and a decrease in k' value. It was considered that the organic molecules are adsorbed on the surfaces of phage particles and function as scavengers of oxidative radicals (OX), and as protectors for the phage surfaces against oxidation by OX. This consideration was supported by the finding that an elevation of reaction temperature (T = 283-313 K) in the presence of glucose resulted in reduction of n value as well as enhancement of k' value.