Interdiffusion processes in the polycrystalline Au (30 nm)/Cu (40 nm) thin films during annealing at 100—200C for 15 and 30 min in a vacuum with different residual-atmosphere pressures of 10$^{-2}$ and 10$^{-6}$ Pa and in an environment of hydrogen at a pressure of 5$\cdot$10$^{2}$ Pa are investigated. Secondary neutral mass spectrometry complemented with transmission electron microscopy, X-ray diffraction, and atomic force microscopy is used. The intermixing of two layers is observed at the temperatures, at which the bulk diffusion can be safely ruled out. Within the Au layer, the detected Cu profiles and their time evolution are typical $C$-kinetic regime grain boundary profiles. Due to the much higher grain boundary diffusivity, the saturation of Au grain boundaries at the Au-rich side is hit in a very short time. The high Cu concentration level in the Au-side could be interpreted by supposing existing boundaries, which are moving and leaving behind themselves the areas with high Cu concentration close to the stoichiometric compositions. Phase formation takes place in and around grain boundaries due to diffusing component transport along moving grain boundaries.