The treatment of a model wastewater containing polyethylene glycol (PEG) of molecular weight 10 000 in an integrated wet air oxidationÈaerobic oxidation process was investigated. Partial wet air oxidation under mild operating conditions was capable of converting the original polymer to lower molecular weight compounds, such as oligomers and short-chain organic acids. The e †ect of molecular weight on the aerobic biodegradability of polyethylene glycol was assessed by performing shake Ñasks experiments with various polyethylene glycols in a molecular weight range from 200È35 000 and it was found that biodegradability generally decreased with increasing molecular weight. Aqueous solutions of PEG 10 000 were subjected to continuous wet air oxidation at a temperature of 423 K and a residence time of 30 min and the oxidised effluents were then subjected to subsequent continuous aerobic oxidation at residence times varying between 0É5 and 4 days. Simultaneously, continuous aerobic oxidation experiments on solutions of PEG 10 000 were performed and used to compare the efficiency of the integrated treatment process with that of the direct biological treatment. Partial pretreatment by wet air oxidation under mild operating conditions resulted in effluents whose biodegradation rates were signiÐcantly higher than those of the original, unoxidised wastewater, as assessed by total organic carbon (TOC) removal rates. The original wastewater was practically non-biodegradable at a 0É5-day residence time with direct biological treatment, while an 80% TOC removal was achieved when biological treatment was coupled with a preceding wet air oxidation step. Conversely, with a 4-day residence time for the direct biological treatment the original wastewater was only partially degradable with 60È70% TOC removal recorded ; with the integrated treatment process at a 4-day residence time in the bioreactor the overall TOC removal was greater than 90%.