Rationale 5-Amino-4-chloro-2-phenyl-3(2H)-pyridazone (Chloridazon) is an important systemic herbicide; its transformation by different Advanced Oxidation Processes could result in the formation of different and complex products. These products, that need to be identified, may present a more toxic effect than the parent compound. Methods The exact and unequivocal structural characterization of the detected by-products of Chloridazon was performed by in-depth analysis of the liquid chromatography/negative electrospray ionization tandem mass spectrometric (LC/ESI+/MS2) fragmentation. Such a technique is a powerful and sensitive analytical tool to study environmental issues. The reactive radicals that induce the degradation of Chloridazon were generated by selective excitation of titanium dioxide as photocatalyst and also by UV photolysis of hydrogen peroxide. Results Several processes are initiated by hydroxyl radical reaction with Chloridazon. Among them (i) hydroxylation of the phenyl group at the ortho, meta and para positions; (ii) a second hydroxylation process as a secondary reaction is also obtained within the phenyl moiety; (iii) scission of the phenyl-N bond through a hydrolysis process; and (iv) a substitution of the chlorine atom by a hydroxyl group. Conclusions LC/ESI-Q-TOFMS appeared to be a valuable and precise tool for structural elucidation of the unknown by-products that were generated during hydroxyl radical reactions with Chloridazon. Several hydroxylated and dihydroxylated isomers were identified together with dechlorinated and bridge opening products. Copyright © 2016 John Wiley & Sons, Ltd.