Thin films of ZrO2 and ZrO2–SiO2 were deposited by reactive dc magnetron sputtering. The optical properties, density, microstructure, and crystalline phase of pure ZrO2 films were found to be a function of deposition rate. In particular, the index of refraction could be varied from 1.77 to 2.13 by increasing the deposition rate from 11 to 720 Å/min. The density of the films increased from 3.9 to 5.8 g/cm3 over the same deposition rate range. Small amounts of SiO2 (10 at. %) stabilized the mixed films in an amorphous phase. A linear relationship between index of refraction and SiO2 content was observed and, as in the case of pure ZrO2, increasing deposition rate resulted in mixed films with higher densities and indices for a fixed SiO2 content. The structure and optical properties of the mixed films remained unchanged with thermal cycling up to 500 °C.