In this article, the segregation of cyclic oligomers at the surface of metallized poly(ethylene terephthalate) (PET) is reported. The diffusion, nucleation, and crystallization aspects of this phenomenon have been investigated by means of several surface and imaging techniques. After annealing some Cu and Au metallized PET foils, the Rutherford backscattering spectrometry (RBS) results seemed to indicate fast diffusion of the metal from the surface into the bulk of the polymer. Depending on the metallization conditions, atomic force microscopy (AFM) showed, after annealing, the nucleation and growth of crystals at the surface. From the literature, it is known that these crystals are mainly composed of cyclic PET trimers. This was confirmed by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) chemical images, which revealed the absence of metal in these crystals. A model in which cyclic oligomer crystals grow on a stable discontinuous metal layer is proposed to explain the results. In order to verify its validity, an RBS simulation tool was developed to take into account lateral inhomogeneities at the surface. In this way, good agreement between the AFM and RBS quantitative data was obtained. Finally, a model for the oligomer diffusion and crystallization process is proposed.