Thin amorphous alumina coatings have been deposited on polyethylene terephthalate (PET) by rf magnetron sputtering in a pure argon plasma. Their microstructure, composition, stresses and adhesion to the PET films have been studied. SEM microstructure study showed a good agreement with Thornton’s structure zone model, i.e. a progression from a dense structure to a columnar structure was observed as the pressure was increased. The composition of deposits, determined by RES, showed that oxygen-rich alumina was obtained when high plasma pressures (more than 1 Pa) were used. FTIR spectra of alumina indicate that the excess oxygen was essentially due to the presence of hydroxyl groups in the coatings. The stress evolution in alumina deposits, determined by the bending beam method, was correlated with the microstructural change. Adhesion of alumina on PET has been studied by a peel test. Best results were obtained when sputtering parameters combined moderate power (1 W cm(-2)) and pressure of about 1 Pa. XPS analysis of the alumina/PET interface showed that bonding between the ceramic and the polymer occurred primarily via Al-O-C bonds.