The thermo-oxidative degradation of poly(ethylene glycol)/poly(L-lactic acid) (PEG/PLLA) blends was studied by infra-red spectroscopy (IR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and thermogravimetry (TGA). The thermo-oxidative degradation of PEG occurred after a period time of aging in air at 80 degreesC. The mechanism of thermo-oxidative degradation of PEG was found to be the random chain scission of the main chain. As PEG blending with PLLA, the existence of PLLA appeared to enhance the thermo-oxidative degradation of PEG. The enhancement of thermo-oxidative degradation increased first and then decreased with the increase of PLLA. The results could be attributed to the ease of abstraction of the carboxylic hydrogen (-COOH) of PLLA, which enhanced the thermo-oxidative degradation of PEG.-Also, the dilution effect of PLLA on the concentration of free radicals was an important factor of the thermo-oxidative degradation. (C) 2003 Elsevier Ltd. All rights reserved.