Polyethylene terephtalate (PET) metallized with aluminium by physical vapour deposition was investigated through classical physical chemistry techniques and mechanical characterization. The amount of aluminium altered the amount of crystallinity of the PET substrate, but appeared unrelated to the mechanical properties obtained with regular tensile test. In contrast, the essential work of fracture (EWF), as obtained with Cotterell tests, permitted to better discriminate the perforation resistance. It is shown that increasing the amount of crystallinity within the PET linearly reduced the EWF.