Polycrystalline-Si/GaAs interfaces have been prepared by depositing hydrogenated amorphous Si (a-Si:H) onto GaAs in a silane plasma at 450°C and annealing at temperatures between 600 and 1020°C. Rutherford backscattering, secondary ion mass spectroscopy, and transmission electron microscopy show that the resulting polycrystalline-Si/GaAs interface is metallurgically stable when the Si is undoped while significant interdiffusion occurs when 12 at. % As is added to the Si. The data are consistent with the Greiner/Gibbons theory [Appl. Phys. Lett. 44, 750 (1984)] that high concentrations of Si diffuse in GaAs in the form of Si-Si substitutional pairs via Ga and As vacancies.