Abstract Gold-based alloys are used as contact materials for GaAs and Ga-Al-As semiconductor devices. Diffusion anneals of gold films on n-type GaAs and Ga 0.7Al 0.3As substrates were carried out under vacuum in the interface could be identified 350–450°C. The presence of the β phase Au 7Ga 2 in the interface could be identified by thin film X-ray diffraction analysis. Scanning electron micrographs revealed the formation of holes and spikes in the gold contact films. The diffusion profiles obtained from Rutherford backscattering analysis could be explained by the combined results of X-ray diffraction and scanning electron microscopy.