Metal oxides of high dielectric constant are candidates to substitute SiO2 as gate dielectric in complementary metal-oxide-semiconductor technology. In this contribution we present the application of X-ray techniques to the study of thermal stability of 5-, 10- and 20-nm-thick HfO2 deposited by atomic layer deposition. Grazing incidence X-ray diffraction spectra are refined by Rietveld analysis. X-Ray reflectivity (XRR) data are fitted in order to evaluate the thickness, the surface roughness and the interface stability upon thermal processing between 300 and 1050 °C. The electronic density values obtained from the scattering coefficients of the crystallographic phase are compared to the values related to the critical angle of the XRR spectra. The thickness and the electronic density of the as grown layer are slightly affected by the thermal treatment. The surface and interfacial roughness are also very stable. The crystallographic ordering evolves starting from the amorphous structure of the as-deposited film towards the polycrystalline monoclinic HfO2 films obtained by annealing at high temperatures. The results are discussed and compared to plan view and cross-sectional transmission electron microscopy images.