The interaction of Cu and Cu3Ge thin films with Si1−xGex (x=0.5) alloy layers epitaxially grown on Si(100) has been studied in the temperature range of 250–400 °C. In this temperature range, Cu reacts with the alloy to form a Cu3(Si1−xGex) ternary phase with an ordered body-centered-cubic crystal structure. The Cu3(Si1−xGex) phase exhibits high-room-temperature (∼150 μΩ cm) and nonmetallic resistivity. However, this ternary phase is not observed and the diffusion of Cu into the alloy is suppressed when Cu is replaced by low resistivity (typically less than 10 μΩ cm at room temperature) ε1-Cu3Ge phase. In contrast to the results reported for films of ε1-Cu3Ge formed on Si(100), the outdiffusion of Si into the ε1-Cu3Ge films is found to be suppressed when the films are formed on Si0.5Ge0.5 layers, indicating the increased stability of ε1-Cu3Ge on Si1−xGex alloys compared to pure silicon.