Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO2/SiOx multilayers. The SiO2(2 nm)/SiOx(2 nm) multilayers are prepared under various deposition temperatures by ion beam sputtering. Annealing at temperatures above 1100 degrees C leads to the formation of Si nanocrystals (nc-Si) in the SiOx layer of multilayers. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentz lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO2/nc-Si multilayers as a mixture of c-Si and SiO2. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiOx layer. This result is compared to expected nc-Si volume fraction, which is estimated from the stoichiometry of SiOx. (c) 2004 Elsevier B.V. All rights reserved.