Bulky high-density polyethylene (HDPE) specimens containing different stabilization systems are exposed in air for up to 7200 h in the melt state to investigate the HDPE's applicability as phase change material (PCM). Their thermo-oxidative stability is investigated. The maintenance of the storage capacity (heat of fusion) and the HDPE's degradation behavior is monitored via Differential Scanning Calorimetry (DSC). Different aging phenomena of the thermo-oxidative exposure are investigated in more detail by means of Fourier-transform infrared spectroscopy (FTIR) and polarized light microscopy (PLM). Only a small loss in the storage capacity upon thermo-oxidative exposure is detected. Therefore, HDPE proves to be a candidate material for polymeric PCM.