Through a direct observation, based on the spin-polarized tunneling technique, we explore the thermal stability of tunneling spin polarization in Al/Al2O3/Co junctions. Thermal robustness of this parameter, which is of key importance for magnetic tunnel junction performance, is established for in situ postdeposition anneal temperatures up to 500 °C. This stability is consistent with detailed in situ x-ray photoelectron spectroscopy measurements on the Al2O3/Co system which show no structural changes during the anneal. Our results imply that, for comparable magnetic tunnel junction devices, thermal stability is not limited by intrinsic processes in the Al2O3 barrier and its interfaces. With ex situ postdeposition annealing in an Ar-atmosphere, which leads to severe degradation of the spin polarization above 250 °C, we demonstrate that the spin polarization is extremely vulnerable to diffusion of impurities.