Aqueous 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) has been oxidized by ozonation, photocatalysis and photocatalytic ozonation. Diuron degradation takes place via radical pathway through hydroxyl radicals in those systems involving ozone. Diuron elimination in photocatalytic ozonation is not enhanced if compared to single ozonation; however, TOC removal was significantly improved. Specifically, 80% TOC removal in 2 h was reached in photocatalytic ozonation while single ozonation just led to 25% TOC reduction. Photocatalysis required 9 h to reach 25% TOC reduction. Ten transformation by-products generated during the application of the three technologies were tentatively identified by liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-QTOF-MS/MS). Single ozonation and photocatalytic ozonation led to the formation and complete elimination of all by-products. Low weight carboxylic acids evolution suggests that high TOC removal in photocatalytic ozonation is linked to its capacity to oxidize small oxygenated compounds and release of inorganic chloride and nitrate. Toxicity evolution to Vibrio fischeri in photocatalytic ozonation displayed an increase in inhibition at the initial stages (>90% of inhibition), followed by a decrease of this parameter as the reaction progressed. The final treated sample shows a lower toxicity than the initial one (55% vs 20%). (C) 2016 Elsevier B.V. All rights reserved.