Amorphous films in the system Al2O3-Y2O3 were prepared by the rf sputtering method in the range of 0-76 mot % Y2O3, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al2O3-Y2O3 films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y2O3 content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al2O3-Y2O3 films, the AlKalpha X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mot % Y2O3 content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y2O3, the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y2O3 content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al2O3-Y2O3, the change of the coordination state of aluminum ions has an important effect on physical properties.