Transformation byproducts and pathways for the degradation of 2-phenylbenzimidazole-5-sulfonic acid (PBSA), a widely used organic active sunscreen agent, by 1 UV254nm/H2O2 advanced oxidation process (AOP) were elucidated by mass spectrometry. Fourteen byproducts were identified, including environmentally hazardous benzamide (BD) and benzamidine (BZD). Beside of aromatic ring opening, desulfonation (-80 Da) and hydroxylation (+16 Da) were also two main reaction pathways. Degradation kinetics were compared with two PBSA structurally related compounds 2-phenylimidazole (2-PIZ) and benzimidazole (BZ) to estimate the application feasibility of the studied AOP. No apparent photolysis was noticed while the degradation of 1 mu M pollutants in the presence of 1 mM H2O2 followed pseudo-first-order kinetics, with their observed rate constant (k(obs)) in the order of 2-PIZ > PBSA > BZ > BD > BZD, which is consistent with their respective second-order rate constant with hydroxyl radical. However, at high pollutant and oxidant concentrations, i.e., 50 mu M and 5 mM, respectively, light attenuation contributed significantly to the change in k(obs) in the following manner, 2-PIZ similar to BZ > PBSA similar to BD > BZD. SO42- and NH4+ were the major inorganic mineral ions monitored in this study. Release of SO42- from PBSA degradation system reached 70% at 190 min. At the same time interval, NH4+ were <50% for all pollutants studied. Ultimate mineralization of PBSA, 2-PIZ, BZ, BD and BZD in terms of %TOC at 190 mM was found to be 23%, 38%, 45%, 57% and 62%, respectively. UV/H2O2 was shown to be efficient for the removal of organic sunscreen agents from water, however, special care toward the formation of environmentally hazardous byproducts should be considered. (C) 2015 Elsevier B.V. All rights reserved.