Among the polycrystalline thin film conductors of Al, Cu, Ag, Au and their alloys, conductors made of Cu alloyed with Be exhibit the longest lifetimes under similar conditons of electromigration testing. Data have been analyzed for a Cu-Al sample tested at 2×1016 A cm-2 and 265°C for 1400 h and for a Cu-Be sample tested at 3×106 A cm-2 and 291°C for 13 000 h. In both cases the transport of Cu occured from the negative to the positive terminal. The rate of grain boundary diffusion of Cu in Cu-Be is more than fifteen times smaller than that in Cu-Al. Observations of mass transport effects due both to electromigration and to compressive stresses in Cu-Al samples indicate that the atomic flux in grain boundary diffusion is accompanied by a counter flux of defects, presumably vacancies. © 1975.