The interface between aluminium and poly(ethylene terephthalate) (PET) has been studied using the density functional theory (DFT) formalism within the local density approximation (LDA). An examination of the charge transfers during Al bonding with the polymer functional groups provides a better understanding of the nucleation of the metallic film. Indeed, when reacting with the doubly-bonded oxygen atom of the ester function, the metal particle induces strong modifications in the highest occupied molecular orbitals (HOMOs) as well as in the lowest unoccupied molecular orbitals (LUMOs) of the PET. The shape of the HOMO, HOMO-3 and HOMO-4 of the Al/PET complex confirms the evolution of the phenyl structure to a quinoid structure. We emphasise the extent to which the reactivity of the PET can be modified after Al interaction with the ester function. After losing their aromaticity, the phenyl rings of the polymer are no longer available for subsequent interactions. Hence, we propose that the Al film will then grow following a compact cubic stacking on top of Al atoms already fixed on the ester functions.