Present paper looks into the possibilities and limitations of near ultraviolet-visible range spectroscopic ellipsometry in investigating HfO2 thin films (thickness < 7 nm). The "high k" dielectric films were produced by Atomic Layer Deposition-ALD, sputtering, and Metalo-Organic Chemical Vapour Deposition-MOCVD, on silicon and on silicon/silicon dioxide structures. Using a simple optical model (Cauchy dispersion, with an Urbach absorption tail), suitable for the optical range investigated, we extract the thickness of the layers and their optical constants. Results related to the optical properties show the important impact made by the initial surface and the growth/deposition procedure. It is also shown that for the case of ALD HfO2 films grown on RTO oxides a significant increase in the absorption coefficient is recorded in the 4.7-5.15 eV range; this can be linked with the formation of defects related to oxygen vacancies. Subsequent anneal cycles performed in oxygen reveal that changes do occur both at the transition layer level, and in the structure of the HfO2 film, for which an increase in the absorption is recorded. (c) 2005 Elsevier B.V. All rights reserved.