Dynamic electrical thermal analysis (DETA) is considered as a valuable technique for determination of polar polymer structure or changes in the polymer structure as a result of different treatments. Therefore, with this study, we wanted to check whether this technique can indicate structural changes in the PET packaging material in contact with specific media. Positive response give the opportunity to use the study of PET packaging dielectric properties after a programmed contact with some medium, to indicate possible interactions between packaging material and the medium, or packaging and foodstuff. It is also known that official simulants may have some drawbacks as migration of the potential contaminants depends on the interaction between the simulant and packaging and thus the values for measured migration could be exaggerated or too low.The possibility of DETA to indicate structural changes in the packaging material give also the opportunity to adjust the aggressiveness of some medium to the packaging, that is, some solvent, or mixture of solvents with different polarity, and thus to choose the most appropriate simulant – medium which will behave in the same way as the foodstuff.In this study we have chosen several conventional fatty food simulants: olive oil, isooctane, 3% acetic acid, and ethanol, and using the DET analysis we compared the influence of these media and the real foodstuff (mayonnaise) on the structure of poly(ethylene terephthalate) (PET) food containers.