The development of biaxial stress in FeAl8 thin films was investigated during annealing in vacuum. The heat treatment leads to irreversible changes in the microstructure of the FeAl8 thin films, which were determined by stress-temperature measurements, transmission electron microscopy, and X-ray diffraction. The stress development is explained by semiquantitative considerations of atomic rearrangement (excess-vacancy annihilation, grain-boundary relaxation, and grain growth). The stress development between 180 and 400 degrees C is affected by grain-boundary relaxation, and between 450 and 675 degrees C by grain growth. The high thermal stability, which was measured in previous investigations, could not be confirmed.