Interdiffusion at the polymide-Cu interface has been studied with high resolution medium energy ion scattering (MEIS) and cross section transmission electron microscopy (TEM). Cu was evaported on polyimide in UHV and studied in situ with ion scattering. In the temperature range from 293 to 593 K Cu is found to diffuse into polyimide and form small spherical particles. The size of these spheres and the depth at which they occur increase with increasing temperature. Once the Cu spheres have been formed they become relatively immobile. It is argued that the kinetics of Cu-polyimide interdiffusion is determined by atomic diffusion of Cu. Chemical effects do not appear to play an important role. © 1985, American Vacuum Society. All rights reserved.