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Oxidation kinetics and degradation pathways of phenazone (an analgesic and antipyretic drug) upon reaction with O 3 were investigated. Kinetic studies on degradation of phenazone were carried out under different operating conditions such as temperature, pH, anions and H 2 O 2 addition. Results showed that the degradation followed the pseudo-first-order kinetic model. The reaction rate constant ( k obs ) of phenazone reached the maximum at 20 °C (9.653 × 10 −3 s −1 ). The presence of NO 3 − could enhance the degradation rate, while the addition of HCO 3 − , SO 4 2 − , Cl − and the rise of pH showed negative effects on the ozonation of phenazone. H 2 O 2 addition increased the phenazone degradation efficiency by 45.9% with the optimal concentration of 0.135 mM. Reaction by-products were evaluated by UPLC-Q-TOF-MS, which allowed the identification of a total of 10 by-products. The transformation pathways of phenazone ozonation consisted mainly of electrophilic addition and substitution, pyrazole ring opening, hydroxylation, dephenylization and coupling. The toxicity of these intermediate products showed that they are expected not to be more toxic than phenazone, with the exception of P7 (aniline) and P10 (1,5-dimethyl-4-((1-methyl-2-phenylhydrazinyl)methoxy)-2-phenyl-1H-pyrazol-3(2H)-one).