In this work, we report on a spectroscopic ellipsometry study of thin films of silicon nanocrystals embedded in a SiOx matrix that is performed to better understand how substoichiometric SiOx species affect the optical properties of these systems. The silicon nanocrystals are fabricated in a SiOx matrix by thermal annealing of magnetron-sputtered thin films in a wide range of x values (0< x< 2) in order to produce every possible substoichiometric SiOx species in various fractions. A generic optical model to describe the thin film content with various SiOx volumetric fractions is developed and used to analyze the data obtained from the spectroscopic ellipsometry. Tauc-Lorentz and Bruggeman effective medium approximation models are employed to determine the ellipsometric angles (psi and Delta) and the complex dielectric function. We show that the optical properties of the nc-Si embedded in a SiOx matrix are highly dependent on the value of x and on the volumetric fractions of the substoichiometric SiOx species present in the system. (C) 2014 Elsevier B.V. All rights reserved.