The influence of Hf doping on the interfacial layer of Ta2O5 stacks was studied by Variable angle spectroscopic ellipsometry (VASE). It was demonstrated new abilities of ellipsometry, beyond the traditional control of thicknesses and optical constants of very thin layers in stacks. An uncommon approach with a proper algorithm for VASE data interpretation was applied to identify the interfacial layer composition, the main interfacial constituents, the elemental depth profiles and its modification due to Hf intervention. In the investigated interfacial layers an inhomogeneous presence of non-transparent Si constituent was detected. A quantitative analysis of Si distribution in IL depth was performed. The depth profiles of other constituents as "effective" Si3N4, Ta2O5 and SiO2 were also retrieved in stacks with nitrided and bare Si substrates. Hf doping of stacks with nitrided substrates strongly affects the interfacial homogeneity by the assistance of nitrogen presence. Moreover, scavenging of SiO2 by Hf and IL thickness reduction were observed. In stacks on bare Si substrate the Hf doping did not produce significant changes. Some comments for the possible reactions were proposed. (C) 2012 Elsevier B.V. All rights reserved.