Different cold plasmas have been used to treat the surface of polyethylene terephtalate (PET) in order to improve the adhesion of alumina thin films deposited by RF sputtering. The influence of these treatments on the surface free energy of the polymer is shown by a study of wettability. ESCA analysis of the PET surface suggests that chemical changes occur as the polymer is plasma treated. The adhesion of alumina films on PET is studied by using tensile testing. The results show that the surface treatment of the PET by a slightly oxidizing plasma, such as carbon dioxide, increases by a factor of 1.7 the adhesion of alumina coatings.