The aging of high-density polyethylene (HDPE) when exposed to drastic climatic conditions (Canadian winter, characterized by low temperature and abrupt temperature variations between the night and the day) was studied. The importance of degradation was determined by evaluating the microstructural changes in HDPE (i.e., oxidations, ramifications, and polymeric chain breaking) by means of FTIR spectrophotometry. The crystallinity variation in HDPE by FTIR and DSC was also studied. Both techniques led to coherent results : there was a loss of crystallinity due to weathering degradation. This crystallinity reduction produced a drastic decrease in impact energy. However, the other properties evaluated were not significantly affected.