The thermal oxidation of Co alloy and Ni alloy thin films on top of Cu, has been studied as a function of film composition. Four different alloys were studied; CoWP, CoWB, CoMoB, and NiMoB. For the Co alloys, significant oxidation is observed after an air anneal above 350C, accompanied by Cu diffusion through the film to the surface. In contrast, for the NiMoB films, there is minimal oxidation of the surface even after a 400C anneal in air, and Cu diffusion through the film is greatly reduced. These results indicate that Ni-based barriers may be desirable in applications where self-aligned metal layers are used on top of Cu interconnects as a replacement for dielectric caps such as SiN and SiCN.