Ultra-thin (1-8 nm) HfO2 films, grown on both chemical and thermal SiO2/Si surfaces by atomic layer deposition, were characterized in terms of interface properties by angle-resolved X-ray photoelectron spectroscopy (ARXPS), high-resolution transmission electron microscopy (HRTEM) and depth profiling techniques such as secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES). ARXPS measurements show, in excellent agreement with HRTEM, that the SiO2 thickness increases upon HfO2 deposition, possibly in part as the result of interfacial reactions forming silicates-type bonding states that are evidenced from careful deconvolution of both Hf 4f and Si 2p core-level spectra. SIMS and AES depth profiles exhibit a redistribution of oxidized Hf concentration in the SiO2 interfacial layer, and also a possible diffusion of Hf into the Si substrate. These results are useful for further understanding of HfO2/SiO2 interfaces for the next generation of complementary metal-oxide-semiconductor transistor oxide gates.