Aluminum oxide thin films were deposited on PET substrate film using r.f. magnetron sputtering with an aluminum oxides magnetron target and argon plasma. The electronic structures of the resulting aluminum oxides were then examined using XPS. The XPS spectrum for the aluminum oxides showed the presence of a well-resolved Al 2p spectral line at 74.3±0.2 eV (FWHM=∼2.1). The binding energies of Al 2p and O 1s for the samples prepared under the synthetic conditions in the current study were the same, or very similar. The average ratio of O_o/Al for all the aluminum oxides was 2.8, thereby indicating that the deviation from the stoichiometric atomic ratio of 1.5 in Al₂O₃ was accounted for by incomplete oxidation. Given reference energies of 531.0 eV for O 1s and 72.7 eV for Al 2p, the Al 2p peak tended to shift to higher energies, while the O 1s peak shifted to lower energies. During the synthesis of the aluminum oxide thin films, there were correlations between the preparative conditions and the relative intensities of the Al 2p and O 1s peaks. The peak intensities of Al 2p and O 1s increased with an increase in both sputtering power and time. These results indicate a linear relationship between the peak intensities of Al 2p and O 1s and the power.