We performed a study of the diffusion of Ag and Au atoms in polyethyleneterephtalate (PET). Thin metal layers were deposited using a diode-sputtering technique on polymer foils at room temperature. Simultaneous post-deposition annealing and plasma treatments were used to induce metal–polymer intermixing. Rutherford back-scattering spectrometry and X-ray photoelectron spectroscopy were used to determine the integral amount of metal and chemical structure in the surface layer. After plasma treatment Ag thin films exhibit dramatic changes of chemical composition and an integral amount of metal compared to Au thin films. Transmission Electron Microscopy shows the differences in the size and the depth distribution of metal particles, depending on the annealing temperature at the metal–polymer interface.