In this study, effluent from the biological treatment of wastewater from a resin-producing factory containing recalcitrant compounds was ozonated under different conditions. Afterwards, the biodegradability of the ozonated effluent was studied under anoxic conditions. The post-ozonation of the industrial effluent was performed using a wide range of ozone doses, from 1.8 to 29.5 mg L-1 min(-1). After the biological treatment of the ozonated effluent, organic carbon and nitrogen removals from 27 to 97% and from 27 to 80%, respectively, were achieved. The effect of the contact time was studied at a constant ozone dose of 13.0 +/- 4.2 mg L-1 min(-1) and contact times ranging from 30 to 180 min. In this case, organic carbon removals from 55 to 100% and organic nitrogen removals from 41 to 77% were obtained after biological treatment. (C) 2006 Elsevier B.V. All rights reserved.