Outdoor degradation of isotactic polypropylene (PPI) plates was studied by means of positron annihilation lifetime spectroscopy (PALS), absorption infrared spectroscopy, differential scanning calorimetry (DSC) and density measurements. Infrared spectra reveal the presence of oxygenated species into the exposed polymer induced by external agents. Results from thermal and density analysis suggest an increase of crystallinity of the sample with exposure time. Positron data strengthen such a conclusion, showing a reduction of the amorphous zones as monitored by the corresponding decrease of positronium (Ps) formation. Furthermore, an estimation of the average sizes of the free volume holes and of the defects in the crystalline regions was obtained.