The adhesion of an alumina coating from 10 to 200 nm thick deposited by r.f. magnetron sputtering on polyethylene terephthalate (bi-stretched; 12 mum thick) has been characterized. Modification of the surface energy of the polymer by a cold CO2 plasma was found to be an excellent method to increase adhesion as characterized by a fragmentation test. Various parameters were studied for the plasma treatment : r.f. power; duration; pressure. X-ray photoelectron spectroscopy analysis revealed the formation of a superficial carbonyl function which involves an increase in the polar component of the surface energy. The presence of these carbonyl groups promotes Al-O-C or Al-C type of bond formation which may be responsible for the good level of adhesion observed.