The treatment of a model wastewater containing polyethylene glycol of molecular weight (MW) 10 000 by means of combined chemical oxidative pretreatment membrane separation and biological post-treatment was investigated. Wet oxidation was employed as a chemical pretreatment process to convert the original, high MW polymer to lower MW compounds in an attempt to improve the biotreatability of the waste-water. The partially oxidized effluents formed during wet oxidation at temperatures up to 403 K were separated by nanofiltration where larger molecules were recycled inco the wet oxidation reactor, while the permeate leaving the filtration step was treated biologically. At a biological residence time (tau(b)) of 12 h and 3 h, the resulting total organic carbon (TOC) removal in the biological step was as high as 94% and 87%. respectively. Conversely, a continuous aerobic biological process was found inadequate to completely mineralize the original wastewater, since at tau(b) of 96 h only about 60% to 70% TOC removal was achieved, while at tau(b) of 12 h the original wastewater was practically while at tau(b) of 12 h th non-bioduegradable.