Positron lifetime measurements have been performed in polypropylene (PP). 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 hours). All the lifetime measurements have been made in air, at room temperature (295-297 K). The degree of 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. Mean free volume radii and fractional free volume have been calculated from the lifetime data.