Coevaporated e‐beam films show modification of structure and properties that depend on composition. In pure zirconia films, x‐ray diffraction indicates two crystal phases present and scanning electron microscopy(SEM) shows a columnar structure. The pore fractions in these films can exceed 25%. The admixture of small amounts of silica (20%, the films show an amorphous structure lacking columnar growth. Measurements of refractive index and thickness as a function of composition indicate an initial decrease in porosity with increasing silica content. A simple model gives the porosity of the films as a function of composition. In addition, the mixed films show greater index stability, decreased surface roughness, and decreased optical scatter.