Titanium dioxide nanoparticles (TiO NPs) are among the most widely manufactured nanomaterials on a global scale. However, prudent and vigilant surveillance, incumbent upon the scientific community with the advent of new technologies, has revealed potentially undesirable effects of TiO NPs on biological systems and the natural environment during their application and discharge. Such effects are likely best evaluated by first assessing the fate of the TiO NPs in natural environments. In this study, the effects of terrestrial humic acid (HA) and tannic acid (TA), two major members of the collective: dissolved organic matter (DOM), on the cytotoxicity of TiO NPs to Escherichia coli were investigated in the presence and absence of natural sunlight. Qualitative (transmission electron microscopy (TEM)) and quantitative (LC) analyses were employed in this study. In addition, the production of reactive oxygen species (ROS) in the form of OH was further assessed-as HA or TA increased the production of ROS decreased. The inhibition of bacterial viability in the light treatment groups, with respective treatment organics at concentrations of 10 ppm, was less in TA than in terrestrial HA. SAS was used to analyze the treatment effect of individual factors of light irradiation, DOM, and concentration of TiO NPs.