The discharge of treated wastewater into natural receiving waters is the final process in wastewater treatment plants. It has been proven that most existing wastewater treatment plants are not effective enough in treating numerous xenobiotics in wastewater, which ultimately leads to their release into the aquatic environment. Therefore, it is essential to monitor the effectiveness of xenobiotic removal and to develop new methods and improve existing wastewater treatment processes. The aim of this study is to monitor the removal of individual xenobiotics using an advanced oxidation process, photolysis with the addition of H2O2, and to detect/identify any degradation/transformation products that may form. The investigation was conducted at three different pH-values (pH = 4, 7, and 10) and three H2O2 concentrations (20, 50, and 100 mM). The removal of xenobiotic solutions at a concentration of 10 mg/L was carried out in a Suntest CPS+ device for simulating sunlight (UV-Vis) over a duration of 300 minutes. HPLC-DAD and LC-MS/MS instrumental analytical methods were used to monitor the removal and to identify new products. A total of sixteen (16) new products of neonicotinoid insecticides were detected and identified, of which only three (3) acetamiprid products are known in the current literature. The other resulting products have not been previously recognized according to the literature review, and in this paper, structural formulas for fourteen (14) out of sixteen (16) detected newly produced products of acetamiprid, clothianidin, and thiacloprid are proposed.