Abstract Helium ion back-scattering has been used to examine the interdiffusion behavior of the thin film copper-gold system. Techniques for distinguishing bulk diffusion and grain boundary diffusion using back-scattering are presented. At temperatures in the range 200°–500°C, the grain boundary diffusion mechanism is shown to predominate in Cu-Au thin films. The back-scattering results suggest a model in which interdiffusion takes place by very rapid saturation of the grain boundaries in the gold film by copper and a slower filling of the copper grain boundaries by gold. The atoms in the grain boundaries then diffuse into the grains by bulk diffusion. In the parent gold film, we suggest that the amount of Cu Au 3 formed near the grain boundaries is uniform in depth. In the parent copper film, more Cu 3Au forms near the original Cu-Au interface than further into the copper film. No evidence was found to suggest the formation of a pure layer of CuAu 3 or Cu 3Au. The interdiffusion and compound formation process is found to be characterized by an activation energy of 1.35-1.5 eV.