SiO2 and TiO2, with their high refractive index contrast, are interesting candidates for the fabrication of graded-index optical filters. In this work, SiO2/TiO2 mixtures were prepared by plasma-enhanced chemical vapor deposition from SiCl4 and TiCl4. By controlling the gas flow ratio, it is possible to obtain coatings with refractive index values between 1.48, for SiO2, and 2.35, for TiO2, and with an extinction coefficient below 10(-4) in the visible and near-infrared regions. The optical properties of the mixtures do not respect the Bruggeman effective medium approximation that supposes two separate phases. Using a combination of x-ray photoelectron spectroscopy, Fourier transform I infrared spectrometry, and elastic recoil detection, we demonstrate that SiO2/TiO2 is a single-phase material. Two separate phases can develop at certain compositions by annealing-induced precipitation. (C) 2004 American Vacuum Society.