Amorphous layer up to 20 µm thick were obtained from bulk diffusion couples between pure copper and Zircaloy 4 after isothermal treatment at 900 • C and slow cooling to room temperature. It has been shown that the Cu-rich liquid exhibit a low mobility and high viscosity, and that the asymetrical diffusion behavior of Zr and Cu together with the interfacial reaction rate constrain the liquid layer and allows to avoid nucleation of intermetallic compounds up to 45 minutes holding time. The Cu-rich liquid layer forming at the holding temperature is stable and grows until another liquid, richer in Zr, appears.