Films of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) each containing 25 wt % of linear low-density polyethylene (LLDPE) were extruded by melt blending in a single screw extruder (L/D ratio = 20: 1). The samples were aged at 55, 70, 85, and 100degreesC for different time periods up to 600 h. The change in molecular structure and formation of oxygenated and unsaturated groups during thermo-oxidative degradation were investigated by infrared spectroscopy. Viscosity-average molecular weight ((M) over bar (V)) was found to decrease at slower rate in the initial hours of ageing, whereas on prolonged ageing, it decreased at a faster rate. In some blend samples, the tensile strength and elongation at break initially increased up to certain hours of ageing, and then decreased while in some blends both tensile strength and elongation-at-break varied differently. Increase in percent gel content supports the formation of cross-links between the molecular chains. Thermal stability of the samples also decreased with ageing time and temperature. (C) 2004 Wiley Periodicals, Inc.