The use of neonicotinoid insecticides has been constantly revised because of their impact on bees, causing their decrease and bee malady. Unfortunately, because of the worldwide differences in pesticide regulation, chlothianidin is still allowed in European Union for greenhouse use and worldwide in some cases without any restictions. Lately, it was detected on soil particles and in raw and drinking waters. The preparation of drinking waters implies different purification processes, including chlorination, ozonation, and UV irradiation and nowadays advanced oxidation processes, including TiO2. The TiO2 photocatalytic degradation of chlothianidin in the presence of oxygen, nitrate, and humic acids was followed by kinetic studies, whereas the photoproducts formed were identified by liquid chromatography/tandem mass spectrometry. The efficiency of different set-ups of the photocatalytic degradation of chlothianidin was evaluated by the identification of photoproducts and bioluminescence inhibition of bacteria Vibrio fischeri. The results indicate that less harmful photoproducts are generated in the samples with added humic acids.