A1 films have been deposited onto GaAs(100) surfaces grown by molecular beam epitaxy (MBE). The crystallographic orientation and surface morphology of the A1 films have been investigated as a function of the GaAs substrate temperature (290 or 670 K) and surface reconstruction. Depending on the deposition conditions either A1(100) or one of two orthogonal A1(100) orientations was observed. No influence of the A1 deposition rate was observed in the interval 0.1-1.5 μm h-1. A complex nucleation, involving nuclei with all three observed orientations simultaneously present, always preceded the single crystal growth. Nucleation occurred at a mean A1 coverage of 0.15-0.5 nm and single crystallinity was achieved only after 10-20 nm of A1 had been deposited. The non-preferentially oriented domains were typically 100 nm wide and they were stable up to 580 K, when they disappeared due to interdiffusion and/or recrystallization. The results are explained as a combination of geometrical and chemical effects imposed by the GaAs surface reconstructions and the diffusion across the interface. © 1982.