Specimens have been prepared by deposition of thin films of gold on {100} substrates of gallium arsenide at 280 °C, annealed up to 30 min at 330 °C, and subsequently examined by transmission electron microscopy. During annealing, polycrystalline thin films of gold, measuring approximately 50 nm in thickness, are transformed into thicker monocrystalline square and/or rectangular patches of nearly pure gold, whose edges are aligned along 〈110〉 directions of gallium arsenide, by a process of hole formation in the gold film and subsequent rapid expansion by surface diffusion. Moreover, the nearly pure cubic phases of gold and gallium arsenide are separated by a hexagonal phase of gold–gallium, which assumes orientation relationships {111}Au∥{011̄0}Au–Ga, 〈110〉Au∥〈0001〉Au–Ga and {011̄0}Au–Ga∥{111}GaAs, 〈0001〉Au–Ga ∥〈110〉GaAs with gold and gallium arsenide, respectively. These relationships correspond to the Kurdjumov–Sachs orientation relationship between fcc and bcc structures, if it is assumed that the hexagonal gold–gallium phase can be represented by a modified bcc structure. This phase is believed to form by a process whereby gold reacts with gallium to form the hexagonal phase on {111} planes of both cubic phases and concomitant release of arsenic in the gaseous phase.