The photo stabilising action of low density polyethylene UV masterbatch in slow cooled and quenched extrudates of 6.0 MFI and 0.2 MFI high density polyethylene obtained at varying processing conditions has been examined using IR spectroscopy to monitor the build-up of volatile carbonylic oxidation products after artificial irradiation. Masterbatch dispersion in extrudates of the two grades of high density polyethylene is assessed by UV microscopy. The results show that although increased shear increases the effectiveness of the UV absorber in the high melt-flow-index polymer while it has no significant effect on the performance of the UV absorber in the low melt-flow-index polymer. Quenching the extrudates significantly increases the time to embrittlement. The dispersion of low density polyethylene UV masterbatch is better in the high melt-flow-index polymer when processed at equivalent extruder screw speeds.