Poly(ethylene terephtalate) (PET) films coated with thermally evaporated aluminium are studied by acoustic microscopy after various thermal treatments. Three types of contrasts are observed on acoustic images which are interpreted in terms of the acoustic signature results. The first type of contrast behaviour is explained by the presence of ceramic particles. The second type of contrast behaviour can be characterized by the modelling of the Al/PET system as being adhesion defects. The formation of cyclic oligomers at the PET film surface during the thermal processing is envoked to explain the third type of contrast behaviour. Changes in films produced by thermal processing can thus be followed by acoustic imaging.