The optical dielectric function of hafnium oxide and hafnium silicate thin films was extracted from spectroscopic ellipsometry measurements. The dielectric function is further used for extracting the film density and also to investigate the relation with composition, deposition method and layer stoichiometry. The density of a hafnium silicate film was found to vary between 5563 and 8223 kg/m(-3), for hafnium content (x) between 0.3 and 0.7. We demonstrate that the density of pure hafnium oxide film depends on the deposition method employed and the chemical nature of the starting surface. Also, we investigate the "zero" frequency contribution to the dielectric constant as a function of hafnium silicate film composition. (C) 2008 Elsevier B.V. All rights reserved