The optical constants of Si/ZnO composite films grown on quartz glass substrates were determined in the spectral range 1.5–5.0 eV by spectroscopic ellipsometry using a rotating-analyzer ellipsometer. The structure of the samples was modeled by a two-phase (substrate–film) model, and the optical functions of the film were parameterized through different effective medium approximations. The results allowed us to estimate the microstructural film parameters, such as film thickness, the volume fractions of each of the constituents, and optical constants.