Abstract The ozonation of nitrobenzene in aqueous solution is investigated by determining COD removal, ozone consumption and product distribution at different degrees of ozonation.Results reveal that complete nitro benzene elimination is achieved at relatively short ozonation times, with no appreciable accumulation of unsaturated intermediates.Oxida–tive consumption of about 2.5 oxygen atoms per mole of absorbed ozone is observed in this ozonation stage. Much longer ozonation times are required to reach complete oxidation of intermediates to oxalic acid and carbon dioxide, with an oxidative consumption corresponding to 1.8 oxygen atoms per mole of absorbed ozone. A reaction scheme is proposed on the basis of the observed product distribution.