In this paper we investigate interface evolution upon annealing in o(2) by means of X-ray reflectivity and spectroscopic ellipsometry on HfO2 and ZrO2 thin films deposited with atomic layer deposition (ALD) on Si (100). The analysis of SE data is made with a common optical model adjusting the layer thickness and the surface and interface roughness, while thickness and interface composition are extracted from XRR data by means of the matrix method. The poor electron density contrast between SiOs and Si as opposed to the strong one between HfO2/ZrO2 and Si makes the extraction of structural data from XRR data extremely difficult. Information on interdiffusion phenomena occurring between the high-kappa layer and the interfacial SiO2 can be obtained, while SiO2 growth is hardly detectable. (C) 2003 Elsevier B.V. All rights reserved.