Ozonation of a commonly used non-steroidal anti-inflammatory drug indomethacin (IM) was studied. Kinetic constants of IM with ozone and hydroxyl radicals were measured at an order of magnitude of 10 M-1 s(-1) and 10(9) M-1 s(-1), respectively. IM was degraded within 7 min under the lowest ozone dose, but TOC removal was only 50% even under the highest ozone dose used in the experiments. Ozone rather than hydroxyl radicals was found to be the main oxidant during reaction, with a contribution rate of 80% under pH 7. Six intermediates were identified by high resolution mass spectrometer. Nitrogen atom, C-C double bond and benzene ring were found to be the main reaction sites. Electrophilic attack or Criegee cyclo-addition were proved to be the most probable pathways at the first step. The formation mechanism of one of the ozone products was first proposed during the experiment, then confirmed by the density functional theory (DFI') calculation. Acetic acid, formic acid and oxalic acid were detected as small molecule organic products. The toxicity change during ozonation was measured by luminescent bacterium with results showing that the toxicity can be reduced to zero when ozone dose was high enough. (C) 2016 Elsevier B.V. All rights reserved.