The detoxification of some types of wastewater such as vinasse requires the combined use of chemical and biological treatments. The chemical treatment (eg chemical oxidation) tries to remove resistant compounds in order to make the waste more biodegradable. For economic and environmental reasons, the oxidant should be as selective as possible. Ozone in an acid medium can be an appropriate choice for this purpose. In this work, a straightforward method for estimating the oxidative selectivity of ozone against the rest of the organic load of the waste is developed. Though the kinetic constants found for the reactions of ozone with phenolic compounds are between 20 and 4 times higher than the reaction of ozone with the rest of the whole organic charge, the high organic load (COD) of vinasse yields low values for the selectivity of the reaction of interest, in any case, the selectivity increases with the increasing concentration of phenols. For the highest phenols concentration relative to the whole organic load (ie 0.6%) a selectivity of approximately 12% is obtained. (c) 2005 Society of Chemical Industry.