In this study, the degradation kinetics and transformation mechanism of oxytetracycline (OTC) by UV-254 nm and UV-254 nm/H2O2 were investigated. The removal of OTC increased with increasing initial H2O2 dosage while excess H2O2 acted as an inhibitor of HO. The observed UV fluence based pseudo first-order rate constant of OTC (k(obs), cm(2) mJ(-1)) decreased while the degradation rate at the initial 13 min (mu M min(-1)) increased with increasing initial OTC concentrations. Presence of different water constituents in the reaction system had a different degree of influence on the degradation efficiency of OTC. Though after 10 h irradiation, there was only a limited elimination of total organic carbon (TOC), successful transformation of OTC was demonstrated by the detection of thirty-one degradation byproducts in the UV/H2O2 system. Potential degradation mechanisms for OTC were proposed exhibiting five main pathways, including hydroxylation (+16 Da), secondary alcohol oxidation (-2 Da), demethylation (-14 Da), decarbonylation (-28 Da) and dehydration (-18 Da). This study indicates that UV-254 nm/H2O2 is an effective technology for the removal of OTC from an aquatic environment. (c) 2015 Elsevier B.V. All rights reserved.