The wet air oxidation of aqueous solutions of polyethylene glycol, a synthetic polymer used in a wide range of applications, has been investigated at temperatures from 383 to 513 K and oxygen partial pressures From 2 to 3 MPa. Molecular weights From 62 (ethylene glycol) to 35,000 were studied with respect to their behaviour under wet oxidation conditions in terms of the intermediate compounds produced and the reaction mechanisms. It was found that the autocatalytic mechanism of thermochemical autoxidation was capable of converting macromolecules to lower molecular weight end-products, such as oligomers and carboxylic acids, at very short reaction times and mild operating conditions. The oxidation of the lower molecular weight products was also studied to aid the mechanistic interpretation. Total oxidation of these compounds to carbon dioxide proved to be difficult since compounds such as ethylene glycol and acetic acid are very resistant to chemical oxidation even under more severe conditions. Assessments of the aerobic biodegradability of polyethylene glycols, before and after wet air oxidation, were performed by measuring BOD5/COD ratios and the results are compared to those reported in the literature. The drawbacks of using BOD/COD ratios as an index of aerobic biodegradability are discussed and implications for integrated chemical-biological treatment are given.