The crystallization of a-Si in a-Si (50-nm) and Au (5-nm) thin-film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a-Si first begins at 130 °C, the Au-Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable Au-Si compounds occur. Dendritically growing islands of poly-Si are observed after heating at 175 °C. If the samples are held at a constant temperature of 175 °C for 10 min, the poly-Si islands coalesce. The formation of poly-Si depends on the diffusion of Au into a-Si and the formation of metastable Au-Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly-Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body-centered-cubic structure with a=5.52 Å and composition Au4Si is suggested for the Au-Si compounds.