Ozonation of an antidepressant fluoxetine (FX) and its principle metabolite product norfluoxitine (NFX) were investigated. Experimental results showed that ozonation rates of FX and NFX varied strongly with pH. The kinetic constants of FX increased by 6 orders of magnitude from 3.0 M-1 s(-1)(protonated FX) to 1.7 x 10(6) M-1 s(-1) (deprotonated FX), and that of NFX increased by 5 orders of magnitude from protonated form (3.9 M-1 s(-1)) to deprotonated form (3.5 x 10(5) M-1 s(-1)). Nine and six transformation intermediates were observed for FX and NFX, respectively. Ozonation pathways were proposed as hydroxylation, demethylation, carbonylation and cleavage of the benzene ring. Defluorination products were detected for FX. Resemblance and differences between oxidation products of FX and NFX could help verify the pathway. Formation and removal of ozonation products were monitored at different pH. Finally, ozonation of both FX and NFX led to formation of three organic acids and three aldehydes. The constitution of organic carbon during ozonation of FX and NFX was analyzed. The analysis showed that carboxylic acids made a great contribution to TOC after 60 min ozonation. Vibrio fischeri experiments were used to assess toxicity evolutions during ozonation. The toxicity decreased after ozonation, but cannot be fully removed. The experiments results proved that the formation of aldehyde byproducts may lead to the obstinate toxicity. (C) 2017 Published by Elsevier B.V.