Abiotic or biotic degradation are just some of the processes to which xenobiotics are subject after reaching the environment, so it is necessary to monitor their fate and behavior in the environment. Which processes will take place depends on the physicochemical properties of the pharmaceuticals as well as the environmental conditions present. During such processes, degradation or transformation products may be formed where the newly formed products may be more toxic than the parent xenobiotic molecule. Therefore, the main goal of the research is to examine significant parameters that affect photolytic degradation, to determine the coefficients of degradation rate and to gain new knowledge about the behavior of xenobiotics in water. In this work, the change in concentration of aqueous solutions of selected xenobiotics was monitored as a function of time at different pH-values, which was caused by decomposition under the influence of artificial sunlight. Chromatographic HPLC-DAD analysis of the samples was used to determine the resulting degradation and/or transformation products. Results showed that albendazole and mebendazole as antiparasitics, then clothianidin as a pesticide, are susceptible to photolytic degradation. The pesticide thiacloprid is less photolytically degradable, whereas acetamiprid and febantel are not photolytically degradable.