Abstract The alloying behavior of gold, Au-Ge and Au-Ge-Ni films on GaAs has been studied after furnace annealing at 350 °C, 425 °C and 450 °C for 15 min, 0.5 min and 15 min respectively. Rutherford backscattering reveals that gold from Au-Ge films diffuses into the GaAs at a much lower temperature than that from gold and Au-Ge-Ni films. Transmission electron microscopy analysis reveals the presence of β-Ga 2O 3 in the 450 °C annealed specimens. In addition to this, the presence of Au 7Ga 2 and metastable γ-(Au-Ge) phases was identified in annealed Au-Ge films. Scanning electron micrographs of annealed samples show black patches which were found to be gold deficient compared with the light-gray areas. The surface morphology of Au-Ge-Ni films after alloying is more uniform than that of Au-Ge films.