A trilayer thin film of amorphous alloys Ni40Zr60/Ni73Zr27/Ni40Zr60 has been deposited on nitrided Si surface, annealed at 200, 250, and 300°C in He, and analyzed by Rutherford backscattering spectroscopy and cross-sectional transmission electron microscopy. Interdiffusion occurred without crystallization. When Ni was the dominant diffusing species, no voids were seen. When both Ni and Zr diffuse, submicron size voids were observed within the alloy. These observations indicated that self-diffusion in the amorphous alloy can be mediated by localized vacancylike defects as well as by nonlocalized free-volume defects. Also, void formation as a result of the slower diffusing species is opposite to Kirkendall effect in diffusion couples of crystalline solids.