Pesticides are chemical compounds used to suppress or control pests that threaten various agricultural crops. At the same time, they can have harmful effects on the environment and many organisms, which is a major problem. Neonicotinoids are a new type of insecticide that is being used with increasing frequency. One of the most commonly used neonicotinoids is acetamiprid, which is characterized by high stability and water solubility. Therefore, it can get into the surface and groundwater. The aim of this work is to determine the reactive species responsible for the successful photodegradation of acetamiprid. For this purpose, the influence of the addition of different compounds that selectively react with certain reactive species was tested. Acetamiprid was used as a model compound because it is a constituent component of a commonly used insecticide that comes under the trade name Mospilan (Sigma Aldrich) with a mass fraction of 20%. Photocatalytic degradation of acetamiprid was tested in a rotating photoreactor under conditions of recirculation of the reaction mixture with an immobilized photocatalyst layer and with a UVA radiation source. TiO2 UVC 4h was used as the catalyst. The efficiency of photodegradation was monitored based on the change of acetamiprid concentration with irradiation time using high-performance performance liquid chromatography. Analysis of the results showed that photocatalytic degradation was more effective than photolytic degradation. Superoxide anion radicals and singlet oxygen play a crucial role in the photocatalytic degradation of acetamiprid.