A preliminary chemical treatment of pentachlorophenol, isoproturon, diuron, alachlor and atrazine pesticide aqueous solutions (all them belonging to the list of priority pollutants of the European Union) based on a combination of ozone and photo-Fenton reagents has been used to generate intermediates of partial degradation that could be more conveniently degraded with a secondary biological treatment. Quantification of the biodegradability and the toxicity of the treated solutions have been carried out in order to ascertain the suitability of the coupling between the chemical and the biological step. Biotox((R)), BOD5/COD and respirometric measurements have been performed. The data obtained with the three techniques point towards a potential coupling for the treatment of isoproturon, diuron, alachlor and atrazine, but not for pentachlorophenol, for which non biodegadable and toxic solutions are obtained after chemical treatment.