Thermal relaxation processes due to strain relaxation and Si/Ge interdiffusion were investigated in pseudomorphic p-type SiGe/Si quantum wells using infrared-polarization-resolved absorption spectroscopy. The samples were annealed from room temperature up to 1060°C and intersubband transitions between the lowest heavy-hole states and inter-valence-band transitions between heavy-hole and spin-split-off hole states were utilized to probe thermal activation processes. The strain relaxation process is activated at temperatures above 750°C and causes a decrease of the intersubband absorption and an increase of the inter-valence-band absorption. At temperatures above 940°C, we found that a second process of Si/Ge interdiffusion causes a reduction of all absorption lines in the spectrum. We proposed a simple model that provides a qualitative explanation to the above results.