Transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to characterise thin metal films (Mg, Al, Cu, Ag) thermally evaporated onto polyethylene terephthalate (PET) and to study the formation of the Al/PET interface. The adhesion was measured with a 180 degrees peel test technique. XPS spectra show that the Al atoms react preferentially with the carboxylic group of the PET and that the Al/PET interface exhibits a pseudo layer-by-layer growth mechanism. Two factors strongly favour the increase of metal/PET adhesion : (1) a PET temperature higher than 100 degrees C during metal deposition (Al, Cu and Ag) and (2) a partial pressure of oxygen higher than 10(-5) mbar for the Al evaporation. Furthermore, atomic metal diffusion tends to increase the adhesion while cluster segregation within the PET skin decreases the metal/PET adhesion.