Wet oxidation and nanofiltration are employed in an integrated system for the treatment of bioresistant industrial wastewaters. The partial oxidation products formed during a brief period of pretreatment in a low pressure WO reactor are separated by nanofiltration, and larger molecules are recycled into the reactor where they undergo further oxidation. Experiments were carried out with polyethylene glycol as a model compound in aqueous solution, representing polymer manufacture wastewater, The results demonstrate that by using a combination of an oxidative and a separative step, a higher selectivity of the overall process toward partial oxidation can be achieved. The permeate leaving the filtration step is mainly composed of short chain organic acids which could be readily biodegraded in a subsequent biological treatment, or discharged if concentrations are low.