Ion-beam mixing of the immiscible Ag–Cu alloy system was investigated using 1.0 MeV Kr ion irradiation at temperatures ranging from 80 to 473 K. Mixing of binary (80 nm Cu/55 nm Ag) and multilayer [(8.5 nm Cu/12.2 nm Ag)×11] samples was characterized by backscattering spectrometry, electrical resistivity, and x-ray diffraction. Below room temperature, the Ag–Cu system is rendered completely miscible by Kr irradiation with the formation of a simple homogeneous phase. For irradiation at 473 K, the system again becomes immiscible, but with enhanced solubilities in the two terminal phases. At intermediate temperatures, two or three phases are formed, a nearly equiatomic phase, and one, and sometimes two, terminal phases. Irradiation at elevated temperatures of samples completely mixed at low temperature led to the decomposition of the near equiatomic phase; the new steady states were nearly the same as those obtained by direct irradiation of an as-deposited sample. Phase formation depended only weakly on the ion flux, although at the higher temperatures the decomposition reaction was favored by lower ion fluxes. The experimental results are explained using a diffusion model based on a competition between cascade mixing and thermally activated demixing. ©1997 American Institute of Physics.