The reliability of copper multilevel interconnections requires good adhesion and the prevention of copper diffusion into the interlevel dielectric. Magnesium is a candidate for an adhesion layer and diffusion barrier for copper due to the high heats of formation of magnesium oxides, fluorides and sulfides and the formation of low resistivity compounds of Mg with copper. An investigation of the interactions in thin films of copper and magnesium has been carried out in the temperature range of 25°C to 500°C. The results of these reactions leading to phase formation in Cu/Mg bilayers deposited on Si3N4 or SiO2 using X-ray diffraction, in situ sheet resistance, and Rutherford backscattering measurements are presented in this paper. It was found that the Mg-rich phase CuMg2 is the first phase to form on annealing to approximately 215°C, followed by the formation of the Cu-rich phase Cu2Mg at about 380°C in the presence of excess Cu.