Abstract Precise characterization of high k gate dielectrics becomes a challenging task due to the very thin thickness (<3–4 nm), which will be needed in the next generation of integrated circuits. Conventional techniques such as spectroscopic ellipsometry (SE) in the UV-visible range becomes difficult to use alone because of the great correlation between thickness and optical indices. To overcome this problem a new versatile instrument integrating SE in the VUV spectral range and grazing X-ray reflectance (GXR) has been developed recently by SOPRA. Both kinds of measurements can be made at the same location on the sample and at the same time. The analysis is made with a common optical model adjusting the layer thickness and the surface and interface roughness on the GXR data and the optical indices and other parameters like surface or interface roughness or inter-diffusion on the SE data. The paper describes some experimental results obtained with this system on ZrO 2, HfO 2 and La 2O 3 films. Results are correlated with other experimental techniques in some cases.