X-ray reflectometry (XRR) is a unique technique for thin film analysis providing information on thickness, density, and roughness. The Versailles Project on Advanced Materials and Standards (VAMAS) project "X-ray reflectivity measurements for evaluation of thin films and multilayers" has the goal to assess the accuracy and precision of thickness, density and roughness as determined by XRR measurements and simulations. Here we present preliminary results from the second round-robin analysis on a TaN/Ta metallization on a silicon substrate. This work demonstrated a good inter-laboratory reproducibility for the tantalum film thickness with a standard deviation of 0.04 nm corresponding to about 0.3%. The use of the auto-correlation function (extracted from the derivative of density profiles obtained via a Fourier transform) was found to provide useful layer thickness measurements from important samples that are too complex for a simple direct simulation and fitting approach. (C) 2008 Elsevier B.V. All rights reserved.