The Au-Si reactions on the silicon substrate were studied by cross-sectional observation in situ in a HVEM. The growth and the coalescence of voids in the Au film and the growth of the amorphous film on the top of the Au film were observed at 473 and 573 K, respectively. The observations suggest that first the intermixing of Au with Si at the interface occurs, and then Si atoms accumulate on the top of the Au film by fast out-diffusion of the Si atoms through the Au film. The rapid and vigorous reactions produce many voids in the Au film, and their growth and coalescence make the boundary between the Au and the amorphous film very rough.