Aluminum oxide (Al2O3) films are used in a variety of applications including optics and microelectronics. We have deposited aluminum oxide films on 5 in. 2 polished fused quartz substrates by the rf sputtering method. It was found that properties of these films such as refractive index and density varied significantly while the deposition conditions were held constant. Films with a high refractive index (n > 1.7) were denser and were not attacked by the chemical solution, while the films with a low refractive index (n < 1.6) were less dense and readily attacked. Films with a high refractive index can be reproducibly obtained when a surface layer of the fused quartz substrates was removed by sputter etch before the film deposition. The amount of sputter etch required depended upon the deposition rate to be employed subsequently. The results are consistent with a model in which a hydrated surface layer on the fused quartz substrates provided a source of water molecules that may migrate into the growing Al2O3 films.