Positron lifetime measurements in polyethylene (PE) have been performed. A conventional slow-fast coincidence lifetime spectrometer with plastic scintillators has been used for the lifetime measurements. The investigated samples have been aged in a Xenotest 250T apparatus for different time (from 0 to 3000 h), All the lifetime measurements have been made in air and at room temperature. The degree of the crystallinity and the melting temperatures of the investigated samples have been estimated using the differential scanning calorimetry (DSC) technique. Additional measurements of the hardness (Brinell) have been performed for them. Mean free volumes radii and fractional free volumes have been calculated from the lifetime data. (C) 2001 Elsevier Science Ltd. All rights reserved.