Diffusion couples combining a h.c.p. Zr90Al10 supersaturated solid solution with a fc.c. Cu64Ni36 solid solution were annealed at 410°C for different times. The reaction at the interface was investigated by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDAX). Cross sectional TEM images and electron diffraction patterns reveal the formation of a non-crystalline layer at the interface between the two solid solutions which grows to a maximum thickness of more than 0.3 μm. Using EDAX, the concentration profiles across the layerswere determined. The glassy layer consists of a Zr-Ni-Al alloy with a small amount of Cu.The concentration profiles reveal that two non-crystalline phases coexist in the diffusion couple. One phase is Ni-rich and the other is Zr-rich. A small gradient of the Ni concentration in the Ni-rich amorphous phase and a steeper gradient in the Zr-rich non crystalline phase indicate that the Ni diffusion constant in the Ni-rich phase is larger than the Ni diffusion constant in the Zr-rich phase. In the late stage of the reaction, the growth of a nanocrystalline layer with an average concentration of Cu 90Nij 0 is observed on the Cu-Ni side of the diffusion couple. Crystallization starts at the Zr-Al side of the diffusion couple.