Research on metal diffusion into polymers and metal-polymer interfaces is mainly stimulated by applications of polyimide in microelectronics packaging and at the chip level. Since copper will increasingly replace aluminum in such applications as a result of miniaturization, the interfacial behavior of copper is of great importance. The authors use combined X-ray photoelectron spectroscopy and transmission electron microscopy to show that copper does not diffuse into polyimide from a continuous film. Strong diffusion of Cu is shown to require extremely low deposition rates, far below those of practical importance.