Population growth leads to an ever-increasing consumption of pesticides because they have a positive effect on health and quality of life. When using pesticides, very small amounts reach the target organisms, while the majority is wasted in the environment and pollutes the components of the environment. In addition to positive effects, pesticides also have a negative effect on the ecology and health of the population, so their removal from the environment, especially from water systems is highly desirable. Wastewater is treated with biological, mechanical-physical and chemical methods, which include advanced oxidation processes that are imposed as an alternative to classical procedures. Advanced oxidation processes include heterogeneous photocatalysis. This thesis investigated the photocatalytic decomposition of imidacloprid in a batch reactor with recirculation of the reaction mixture. UVA-LED modules with a power of 30 W were used as a source of photoinduced excitation of the photocatalyst and immobilized TiO2 was used as a photocatalyst. The aim of this work is to determine the reactive oxygen species formed during the photocatalytic reaction, which include superoxide anion radical (O2∙−), hydrogen peroxide (H2O2), singlet oxygen (1O2) and hydroxyl radical (OH∙). For this purpose "scavengers" were used, which have a high reactivity with certain oxygen and other reactive species. It was found that their addition reduces the photocatalytic degradation of imidacloprid. The results show that hydroxyl radical, superoxide anion radical and singlet oxygen play an important role in the mechanism of photocatalytic degradation of imidacloprid. With the addition of potassium persulfate, increased photocatalytic degradation of imidacloprid occurs, which is a consequence of the formation of sulfate radicals which are strong oxidizing agents. A kinetic analysis was performed assuming that the reactions are described by pseudo-first-order kinetics.